Make RTSP-MS-over-UDP negotiation work. See "[PATCH] RTSP-MS 8/15: fix
RTSP-MS UDP" thread on mailinglist.

Basically, UDP setup needs to be done in a particular order (first rtx
on two UDP ports (one for RTP, one for RTCP), then the other streams over
one, single port for all of them together). Not doing this correctly results
in a "461" error (invalid transport) during setup.

Originally committed as revision 17777 to svn://svn.ffmpeg.org/ffmpeg/trunk
