Handle "throttling" in a single function

ngIRCd uses "command throttling" and "bps throttling" (bytes per second).
The states are detected in different functions, Conn_Handler() and
Read_Request(), but handle the actual "throttling" in a common function:
this enables us to guarantee consistent behavior and to disable throttling
for special connections in only one place, eventually.
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>.