sip_session: stop show "incoming" message when the session is cancelled

darcs-hash:20090324141927-b2873-eb201a295f6ae7ca9f01d3ce70cada69c2022366
sip_session: don't register if bonjour

darcs-hash:20090324143345-b2873-a4c23576d37f4b405aa3dad939330207e98b1647