Create local &SERVER channel and log server messages to it

Create local &SERVER channel and log server messages to it

ngIRCd now creates a server-local channel &SERVER with channel modes
+mnPt (moderated, no messages from outside the channel, persistent and
with the topic locked) and logs all the messages to it that a user with
mode +s ("server messages") receives.

If an IRC operator withdraws the +P ("persistent") mode and the &SERVER
channel is freed because of no members, nothing special happens. The
channel can be recerated any time later and ngIRCd would begin logging
to it again.