Set bind2 resource prefix and SASL 2 user agent
Bind 2 does not allow client-generated resource identifiers anymore. It is possible to set a resource prefix hint for the server though. The server might use this as a prefix when generating a resource identifier.
The SASL 2 user agent can be useful to identify open client sessions. The information is not exposed by the server.
The code is only compiled in when QXmpp 1.7/1.8 is available, so this can be merged right away without waiting for the releases and CI updates.