chanserv/quiet: Disallow mixing quiet and unquiet in one command.

This is undocumented and makes it harder to prevent flooding
because the same user might be quieted and unquieted repeatedly.
chanserv/quiet: Also notify target user if we just -v'ed or -e'ed.

The QUIET command can do three things to the target user: devoice, add a
quiet matching them and remove ban exceptions matching them. A notification
should be sent when anything happened, not only when a quiet was added.
chanserv/antiflood: Handle NULL from place_quietmask().