[IRC] Support non-RFC channel privileges.

Add support for non-RFC channel privilege modes such as owner, channel admin
and halfop to the IRC engine and the GNOME frontend.
[IRC] Support usermask channel modes +I and +e.

Add support for the usermask channel modes +I (user may enter an +i channel
without being explicitly invited) and +e (exception from the ban list).

This adds the commands /banexcept, /unbanexcept, /inviteexcept and
/uninviteexcept.