irc: handle AUTHENTICATE as a normal command with server prefix
The previous code expected it to be at the beginning of the line with no
:prefix. Probably because the sasl-3.1 spec gave examples that way.
This adds it as a normal command handler as it should have been since the
beginning. Fixes login to oragono servers.
Don't know if the signature of irc_msg_auth() is API so did it the safe way.