Implement XEP-0233: XMPP Server Registration for use with Kerberos V5.

This specifies how the client determines the hostname of the server for
SASL purposes. There is no specification for mechanisms other than
Kerberos, and while Cyrus SASL requires a non-NULL server FQDN, it
doesn't seem to be important what is passed for the other mechanisms. To
simplify things, use the original domainpart and not the SRV hostname.