deprecate AV_CODEC_ID_VOXWARE and introduce AV_CODEC_ID_METASOUND instead

Voxware is the name of company, it has produced several audio codecs e.g.
MetaVoice family and MetaSound.
rtsp: Don't include the listen flag in the SDP demuxer flags

It's only relevant for the RTSP demuxer. Similarly, the custom_io
flag is only present in the SDP demuxer options list.

Signed-off-by: Martin Storsjö <martin@martin.st>