mod_sasl2: Pull user-agent info into sasl_handler for later reference It may be of interest to post-auth things. Putting it on the session was another option considered, but that seemed unnecessary overhead for something that might be rarely used. sasl_handler is cleared after successful authentication.

This module helps make BOSH and WebSocket connection endpoints
discoverable via the HTTP method described in
[XEP-0156](https://xmpp.org/extensions/xep-0156.html#http).