this patch extends BareosFdPluginBaseclass to provide a standard way to
work with plugin configuration files.

When the plugin options parser encounters one of the options
defaults_file or overrides_file, it will treat the values as a path to
an ini-style configuration file and will try to read it. The resulting
options are added with precedence overrides_file, plugin definition,
defaults_file.
The code will consume the defaults_file and overrides_file options, so
the plugin will not see them later.

(cherry picked from commit de1dd5b)

(cherry picked from commit 91d0754)

by adding a transform (currently only "enc") to an option name, the
python baseclass will now decode the value.
i.e. if you configure `password#enc=E+*g/GAhM4` you will end up with
the option password containing the value "password".

(cherry picked from commit 82193f5)

the dictionaries in the module were previously filled with arbitrary
numbers that matched what we had in the enums.
With this patch we now export the enum value itself, so no mismatches
can happen.
The patch also converts the filetype defines into an enum.

(cherry picked from commit 5082c11)

(cherry picked from commit 387d6b9)

We now encode with b85encode instead of a85encode. The major difference
is the set of characters used to represent the result. While a85encode
will use a colon (:) that is also used as an option separator in the
plugindef, b85encode will not.
As a result values encoded with b85encode will be simpler to use.

(cherry picked from commit 4b2e151)

(cherry picked from commit c9ee9b6)

(cherry picked from commit 62a3b07)