Don't advertise FT ContactCapability unless contact can stream data

Submitted by Simon McVittie

Assigned to Telepathy bugs list

Link to original bug (#23777)

Description

From my caps-sanity branch:

 /* FIXME: should we check for SI, bytestreams and/or IBB too? */
 resource = gabble_presence_pick_resource_by_caps (presence,
     gabble_capability_set_predicate_has, NS_FILE_TRANSFER);

I think we should check for the actual requirements for file transfer (currently I believe that means bytestreams, SI and either IBB or SOCKS5); otherwise we'll falsely advertise the FT capability for people we'd actually be unable to make a stream to.

However, I could be wrong, in which case please close this as INVALID.