This commit just adds documentation for the type flags. The flags are actually
implemented in the following two commits.

… startup

This commit implements support for new ALLOW_BOOL, ALLOW_INT, ALLOW_STRING, and
ALLOW_LIST flags by validating settings with these flags earlier on startup and
providing detailed error messages to users.

The new flags implement stricter error checking than ALLOW_ANY. For example, a
double negated option like -nosetting=0 is treated like an error instead of
true, and an unrecognized bool value like -setting=true is treated like an
error instead of false. And if a non-list setting is assigned multiple times in
the same section of a configuration file, the later assignments trigger errors
instead of being silently ignored.

The new flags also provide type information that allows ArgsManager
GetSettings() and GetSettingsList() methods to return typed integer and boolean
values instead of unparsed strings.

The changes in this commit have no effect on current application behavior
because the new flags are only used in unit tests. The existing ALLOW_ANY
checks in the argsman_tests/CheckValueTest confirm that no behavior is changing
for current settings, which use ALLOW_ANY.

…LLOW flags

Update GetArg, GetArgs, GetBoolArg, and GetIntArg helper methods to work
conveniently with ALLOW_BOOL, ALLOW_INT, and ALLOW_STRING flags.

The GetArg methods are convenience wrappers around the GetSetting method. The
GetSetting method returns the originally parsed settings values in their
declared bool/int/string types, while the GetArg wrappers provide extra
type-coercion and default-value fallback features as additional conveniences
for callers.

This commit makes two changes to GetArg, GetArgs, GetBoolArg, and GetIntArg
helper methods when BOOL/INT/STRING flags are used:

1. GetArg methods will now raise errors if they are called with inconsistent
   flags. For example, GetArgs will raise a logic_error if it is called on a
   non-LIST setting, GetIntArg will raise a logic_error if it is called
   on a non-INT setting.

2. GetArg methods will now avoid various type coersion footguns when they are
   called on new BOOL/INT/STRING settings. Existing ALLOW_ANY settings are
   unaffected. For example, negated settings will return "" empty strings
   instead of "0" strings (in the past the "0" strings caused strangeness like
   "-nowallet" options creating wallet files named "0"). The new behaviors are
   fully specified and checked by the `CheckValueTest` unit test.

The ergonomics of the GetArg helper methods are subjective and the behaviors
they implement can be nitpicked and debated endlessly. But behavior of these
helper methods does not dictate application behavior, and they can be bypassed
by calling GetSetting and GetSettingList methods instead. If it's necessary,
behavior of these helper methods can also be changed again in the future.

The changes have no effect on current application behavior because the new
flags are only used in unit tests. The `setting_args` unit test and ALLOW_ANY
checks in the `CheckValueTest` unit test are unchanged and confirm that
`GetArg` methods behave the same as before for ALLOW_ANY flags (returning the
same values and throwing the same exceptions).

Co-authored-by: Hodlinator <172445034+hodlinator@users.noreply.github.com>

The type flags aren't currently used to validate or convert settings in the
settings.json file, but they should be in the future. Add test to check current
behavior that can be extended when flags are applied.

Co-authored-by: Hodlinator <172445034+hodlinator@users.noreply.github.com>

Let ALLOW_STRING and ALLOW_INT flags be combined with ALLOW_BOOL so string and
int options can be specified without explicit values. This is useful for
imperative settings that trigger new behavior when specified and can accept
optional string or integer values, but do not require them. (For examples, see
the example_options unit test modified in this commit.)

This change has no effect on behavior, and is basically just a documentation
change at this point. The ALLOW_LIST flag is currently ignored unless
ALLOW_BOOL, ALLOW_INT, or ALLOW_STRING flags are also present, and these flags
are not used yet.

-BEGIN VERIFY SCRIPT-
for f in `git grep -n 'GetArgs(' | grep -v _tests | sed -n 's/.*GetArgs("\([^"]\+\)".*/\1/p' | sort -u`; do
   git grep -l -- "$f" | xargs sed -i "/AddArg(\"$f[=\"]/ s/ArgsManager::ALLOW_ANY/& | ArgsManager::ALLOW_LIST/g"
done
-END VERIFY SCRIPT-

- Remove ALLOW_LIST flag from bitcoin-wallet -wallet and -debug arguments. They
  are list arguments for bitcoind, but single arguments for bitcoin-wallet.

- Add ALLOW_LIST flag to -includeconf arg (missed by scripted diff since it's
  not accessed through GetArgs)

- Add ALLOW_LIST flag to -proxy, -debug, -loglevel, -whitebind, and -whitelist
  args (missed by scripted diff due to line breaks in AddArgs calls)

- Add ALLOW_LIST flag to -zmq args (missed by scripted diff due to programmatic
  GetArgs calls)

This change has no effect on behavior, and is basically just a documentation
change at this point. The ALLOW_LIST flag is currently ignored unless
ALLOW_BOOL, ALLOW_INT, or ALLOW_STRING flags are also present, and these flags
are not used yet.

Previous behavior was inconsistent: if -blockfilterindex or
-blockfilterindex="" arguments were specified they would normally enable all
block filter indexes, but could also trigger "Unknown -blockfilterindex value"
errors if followed by later -blockfilterindex arguments.

It was confusing that the same -blockfilterindex options could sometime trigger
errors and sometimes not depending on option position. It was also confusing
that an empty -blockfilterindex="" setting could enable all indexes even though
indexes are disabled by default.

New behavior is more straightforward:

- -blockfilterindex and -blockfilterindex=1 always enable indexes
- -noblockfilterindex and -blockfilterindex=0 always disable indexes
- -blockfilterindex="" is always an unknown value error

The meaning of these options no longer changes based on option position.

Upcoming commits will make it an error to call GetArg and IsArgSet methods on
list options since these usages are error prone. For example GetArg will return
last command line value but first config value in the list, and IsArgSet will
return true even if the list is empty if the list was negated.

This change is just a refactoring replacing problematic ArgsManager calls with
equivalent calls to avoid changing any behavior. Current behavior could
probably be improved in these cases, but this change should make new problems
less likely to be introduced.

Prevent GetArg() from being called on ALLOW_LIST arguments, and GetArgs() from
being called on non-list arguments.

This checking was previously skipped unless typed INT/BOOL/STRING flags were
present, but now it's always done.

This change has no effect on external behavior. It is just supposed to enforce
internal consistency and prevent bugs caused by using the wrong GetArg method
to retrieve settings.

Enable error "Multiple values specified for -setting in same section of config
file.", for ALLOW_ANY settings that don't specify ALLOW_LIST.

Instead of silently ignoring settings, this change makes it an error to provide
an ambiguous config file that provides assigns multiple values to a
single-value setting. Change include release notes.

…ied" errors

Also skip calling TestArgString in these cases since exact behavior retrieving
values is irrelevant when parsing values fails.

Also s/TestArgString/GetArg/ to be more efficient and direct, now that it's no
longer necessary to call with separation of ALLOW_LIST and non ALLOW_LIST
cases.