sip_session: rename --trace-engine to --trace-notifications

darcs-hash:20090407101551-b2873-60c79b8aa29e71c7fae177eb0ea1631dc2f8cc55
sip_session: add duration to "Session ended .." message

darcs-hash:20090407101655-b2873-0bd959a7bf3ecfe725aeb34598554c3c6cd63327