This took a bit of trouble shooting, but gsasl does all the work which is very
nice. I cleaned up some logic around the server_in buffer as well, and we may
need to lock down the logic a bit in purple_ircv3_sasl_authenticate a bit, but
everything seems fine for now.

Use the SASL mechanisms account option to force PLAIN and SCRAM-SHA-256 on my local ergo instance and verified via ngrep that only those mechs were used.
I also deleted the values from the SASL mechanisms account option and verified it fell back to PLAIN that the server advertises. ergo doesn't advertise SCRAM-SHA-256 for some reason, which is why this needed to depend on /r/2222.