Fill s= line with user agent string to fool incompliant endpoints

darcs-hash:20090429112140-58103-846bc7f0003285b9a9c9090372db8a77ff3025bd
Make Session.reject() and end() accept a SIP response code

darcs-hash:20090429112801-58103-390b1833c323740a3626b4c22cc915fc3d22883a