Interpret incoming Entity Capabilities information (XEP-0115)

Look for capabilities information in incoming presence stanzas, retrieve
and verify disco info as needed, and plug into "normal" disco info
cache.
Merge jabber-newdisco into jabber-disco

The functionality is about to start overlapping.  Also, there wasn't
much sense in separating the two to begin with.
Look for caps only on "available" presence
Send Entity Capabilities in outgoing presence stanzas

Also changed the disco info feature "plugin" system - now a module that
wants to advertise a feature needs to call the function
`jabber-disco-advertise-feature'.  This ensures that caps are
recalculated as needed.
Advertise RTT feature
Add jabber-rtt to Makefile.am