avformat/rtsp: add satip_raw flag to receive raw mpegts stream

avformat/rtsp: add satip_raw flag to receive raw mpegts stream

This can be used to receive the raw mpegts stream from a SAT>IP
server, by letting avformat handle the RTSP/RTP/UDP negotiation
and setup, but then simply passing the MP2T stream through
instead of demuxing it further.

For example, this command would demux/remux the mpegts stream:

    SATIP_URL='satip://192.168.1.99:554/?src=1&freq=12188&pol=h&ro=0.35&msys=dvbs&mtype=qpsk&plts=off&sr=27500&fec=34&pids=0,17,18,167,136,47,71'
    ffmpeg -i $SATIP_URL -map 0 -c copy -f mpegts -y remux.ts

Whereas this command will simply write out the raw stream, with
the original PAT/PMT/PIDs intact:

    ffmpeg -rtsp_flags satip_raw -i $SATIP_URL -map 0 -c copy -f data -y raw.ts

Signed-off-by: Aman Karmani <aman@tmm1.net>