Fixes redis#7923.

This PR appropriates the special `&` symbol (because `@` and `*` are taken),
followed by a literal value or pattern for describing the Pub/Sub patterns that
an ACL user can interact with. It is similar to the existing key patterns
mechanism in function (additive) and implementation (copy-pasta). It also adds
the allchannels and resetchannels ACL keywords, naturally.

The default user is given allchannels permissions, whereas new users get
whatever is defined by the acl-pubsub-default configuration directive. For
backward compatibility in 6.2, the default of this directive is allchannels but
this is likely to be changed to resetchannels in the next major version for
stronger default security settings.

Unless allchannels is set for the user, channel access permissions are checked
as follows :
* Calls to both PUBLISH and SUBSCRIBE will fail unless a pattern matching the
  argumentative channel name(s) exists for the user.
* Calls to PSUBSCRIBE will fail unless the pattern(s) provided as an argument
  literally exist(s) in the user's list.

Such failures are logged to the ACL log.

Runtime changes to channel permissions for a user with existing subscribing
clients cause said clients to disconnect unless the new permissions permit the
connections to continue. Note, however, that PSUBSCRIBErs' patterns are matched
literally, so given the change bar:* -> b*, pattern subscribers to bar:* will be
disconnected.

Notes/questions:
* UNSUBSCRIBE, PUNSUBSCRIBE and PUBSUB remain unprotected due to lack of reasons
  for touching them.

…nels permissions by default. (#8723)

Background:
Redis 6.2 added ACL control for pubsub channels (#7993), which were supposed
to be permissive by default to retain compatibility with redis 6.0 ACL. 
But due to a bug, only newly created users got this `acl-pubsub-default` applied,
while overwritten (updated) users got reset to `resetchannels` (denied).

Since the "default" user exists before loading the config file,
any ACL change to it, results in an update / overwrite.

So when a "default" user is loaded from config file or include ACL
file with no channels related rules, the user will not have any
permissions to any channels. But other users will have default
permissions to any channels.

When upgraded from 6.0 with config rewrite, this will lead to
"default" user channels permissions lost.
When users are loaded from include file, then call "acl load", users
will also lost channels permissions.

Similarly, the `reset` ACL rule, would have reset the user to be denied
access to any channels, ignoring `acl-pubsub-default` and breaking
compatibility with redis 6.0.

The implication of this fix is that it regains compatibility with redis 6.0,
but breaks compatibility with redis 6.2.0 and 2.0.1. e.g. after the upgrade,
the default user will regain access to pubsub channels.

Other changes:
Additionally this commit rename server.acl_pubusub_default to
server.acl_pubsub_default and fix typo in acl tests.

…nels permissions by default. (redis#8723)

Background:
Redis 6.2 added ACL control for pubsub channels (redis#7993), which were supposed
to be permissive by default to retain compatibility with redis 6.0 ACL. 
But due to a bug, only newly created users got this `acl-pubsub-default` applied,
while overwritten (updated) users got reset to `resetchannels` (denied).

Since the "default" user exists before loading the config file,
any ACL change to it, results in an update / overwrite.

So when a "default" user is loaded from config file or include ACL
file with no channels related rules, the user will not have any
permissions to any channels. But other users will have default
permissions to any channels.

When upgraded from 6.0 with config rewrite, this will lead to
"default" user channels permissions lost.
When users are loaded from include file, then call "acl load", users
will also lost channels permissions.

Similarly, the `reset` ACL rule, would have reset the user to be denied
access to any channels, ignoring `acl-pubsub-default` and breaking
compatibility with redis 6.0.

The implication of this fix is that it regains compatibility with redis 6.0,
but breaks compatibility with redis 6.2.0 and 2.0.1. e.g. after the upgrade,
the default user will regain access to pubsub channels.

Other changes:
Additionally this commit rename server.acl_pubusub_default to
server.acl_pubsub_default and fix typo in acl tests.