[Pidgin] #16269: irc: should make use of server-sent SASL mechanism list

[Pidgin]

#16269: irc: should make use of server-sent SASL mechanism list
-------------------------+-----------------
 Reporter:  grawity      |      Owner:  elb
     Type:  enhancement  |     Status:  new
Milestone:               |  Component:  IRC
  Version:  3.0.0hg      |   Keywords:
-------------------------+-----------------
 When SASL authentication fails due to an unknown mechanism, some new
 servers send a 908 message containing the list of supported mechanisms:

 {{{
 :test.local 908 grawity PLAIN,EXTERNAL,DH-BLOWFISH,DH-AES,SCRAM-SHA-1 :are
 available SASL mechanisms
 }}}

 The irc plugin should use this list to avoid trying a lot of mechanisms
 that the server doesn't support.

 (The list is unfortunately sent only ''after'' the first failed attempt,
 but that's fine; it still gets rid of many roundtrips considering how many
 mechanisms libsasl supports nowadays.)

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Ticket URL: <https://developer.pidgin.im/ticket/16269>
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