if invite fails, do BYE transaction in the background

darcs-hash:20090123094618-b2873-2660673c8a57457f07eacc6490478ca9a40fcdb1
sip_im_session: in the final shutdown, start all the BYE/UNREGISTER transactions in parallel. This also fixes ctrl-c to interrupt after a single keypress

darcs-hash:20090123094757-b2873-24acd6f436d8e3eb8dbb96f4573f273992b6fe3a