[Pidgin] #88: Jabber Cyrus SASL code should have optional passwords

[Pidgin]

#88: Jabber Cyrus SASL code should have optional passwords
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 Reporter:  sxw  |        Type:  patch    
   Status:  new  |    Priority:  major    
Milestone:       |   Component:  libpurple
  Version:  2.0  |    Keywords:           
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 This is a revised version of the patch in the old SourceForge tracker as
 number 1668289, it's been updated to be against the pidgin codebase.

 Some Cyrus SASL mechanisms, such as GSSAPI, can operate without passwords.
 However, just making passwords optional for the Jabber protocol breaks
 those mechanisms which _do_ require passwords.

 This patch implements an on-demand password prompter. It switches the
 Jabber protocol to make passwords optional, and then adds code to prompt
 for passwords as they are required by the various mechanisms. There is a
 small change to the Gaim core to expose the password prompter function.

 The code takes advantage of the fact that if a password callback isn't
 registered with Cyrus, it won't attempt to use any mechanisms that require
 passwords. So, if we don't have a stored password, we don't register that
 callback, and try to start a SASL session. If we don't have any mechanisms
 available, or all of the available mechanisms fail, we prompt the user for
 a password before trying again.

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Ticket URL: <http://developer.pidgin.im/ticket/88>
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