Remove "choked" check

This hasn't worked since the switch to multi-account, and apparently
hasn't been missed.
Remove *jabber-connected* variable

Obsolete; check whether jabber-connections is non-empty.
Remove *jabber-connection* variable

Obsolete; use jabber-connections instead.
Remove *jabber-authenticated* variable
Validate additional data from server in SASL authentication

Some SASL mechanisms, e.g. SCRAM-SHA-1, specify that the server should
send additional data along with the success response.  This data needs
to be validated by the client according to the SASL mechanism.