Implement user mode "c": receive connect/disconnect NOTICEs
Users having the user mode "c" set receive NOTICE messages on each
new client connection to the local server as well as disconnects.
Only IRC operators (users having the mode "o" set) are allowed to
set the 'c' user mode.
These connect/disconnect messages can be useful for open proxy
scanners -- BOPM (http://wiki.blitzed.org/BOPM) is now functional
with ngIRCd, for example.