sip_session2: fix logic error in update_streams()

darcs-hash:20090423085940-b2873-13e277d72ad543a3796c92a2580a66ca2e099e98
sip_session2: restore cmd_call function and use it when starting a command from the command-line

darcs-hash:20090423090022-b2873-89ba8c5efa233bc0578da4d877a985d79e579aee