irc-channel: implemented a special mode for show_users
This allows for users to be declared as being special, which does not
have any specific meaning. The meaning of being special is different
from protocol-to-protocol, which many protocols do not even implement.
This functionality is mainly geared towards a special user state which
only some protocols may actually need to define. For example, with the
third-party Steam plugin, this can be used for denoting a user which is
actively playing a game.
By default, this mode will not actually be used by any plugin. However,
it does default to the half-operator user mode.
