Adapt to ObjFW changes
XMPPConnection: Verify origin of IQ responses
XMPPRoster: Ensure roster pushes were sent by the server
XMPPConnection: Don't negotiate a session feature, when marked optional

The session feature has been deprecated as of RFC 6120.
Some servers still offer it for client compatibility, but
mark it as optional. Not negotiating it in this case
saves us a round trip
Update buildsys