IRC correctness checks

[Mark Doliner]

Ok, this one is done. Thanks, Tomasz!

I filled in req_cnt for irc_msg_kick and irc_msg_nick and I added back
in some g_return_if_fail(gc) checks. They might not be required when
calling purple_notify_error, but if we expect them to be non-null (I
do) and if the old code required them to be non-null (it did) then it
seems better to keep the old behavior. I think in main we should check
this in parse.c instead of in each msg function.