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.
#!/bin/sh
METHOD_NAME=$1
iftest-z"$METHOD_NAME"
then
cat<<EOF
This program calls purple API functions using DBus and prints the return value.
If you are not interested in the return value, use purple-send-async.