Parse the ETag from the describe method and pass the sessionid as the value for
the If-Match header is subsequent setup calls.
Fixes support for more RealMedia RTSP streams.
Original commit message from CVS:
* gst/realmedia/rtspreal.c: (rtsp_ext_real_before_send),
(rtsp_ext_real_parse_sdp), (rtsp_ext_real_stream_select):
* gst/realmedia/rtspreal.h:
Move assembly rule parsing to the place where we parse the SDP as it's
also there that we create the MDPR and we need the currently selected
asmrule in order to select the right MTLI.
Fixes#529359.