sapenc: Use the normal URL port as RTP port

sapenc: Use the normal URL port as RTP port

This makes the URL handling consistent - the URL hostname is used as
destination for the RTP packets, so the URL port should also belong to that.
Now the options announce_addr= and announce_port specify where the
announcements are sent.

Originally committed as revision 25424 to svn://svn.ffmpeg.org/ffmpeg/trunk