Stop ringtone in Session as soon as audio is started

darcs-hash:20090428140505-58103-8d3a99a5aa52f56674ebe191e5115d7d6e4bab44
session2: update remote_user_agent in outgoing case as well

darcs-hash:20090428145831-b2873-91ed1cf17e5ba35dd4d05b29ed3f960a0aceba9c