chanserv/clear_akicks: add new CLEAR AKICKS command

This command clears all AKICK entries (or access list entries whose
only flags are +b) on a channel (requires +Rf to execute).
chanserv/clear_akicks: replace FLAGS with AKICKS in messages
Merge pull request #345 from lstarnes1024/master+clearakicks

chanserv/clear_akicks: add new CLEAR AKICKS command