Keep track of registration status in sip_message, only print once

darcs-hash:20090417112255-58103-eacc25b63f06f80e57722555972bea43b2caf053
sip_session2: nicer formatting of to_uri in the prompt

darcs-hash:20090417113551-b2873-82a5e7c73c5d18cd94bcfdb60227644dca55b0b1