Add support for owner (~, +q) and admin (&, +a) non-RFC user statuses.

Add support for the non-RFC channel user privilege statuses "owner" (nickname
prefix ~, channel mode +q) and "admin" (nickname prefix &, channel mode +a).
These are most commonly provided on UnrealIRCd-hosted servers, much like "halfop" (nickname prefix %, channel mode +h).