Implement user mode "F": "relaxed flood protection"

ngIRCd relaxes its flood protection for users having the user mode "F" set
and allows them to rapidly send data to the daemon. This mode is only
settable by IRC Operators and can cause problems in the network -- so be
careful and only set it on "trusted" clients!

User mode "F" is used by Bahamut for this purpose, for example, see
<http://docs.dal.net/docs/modes.html#4.9>.