mpegts-mythtv.c: harmonize ff_parse_mpeg2_descriptor() part 1

This is in preparation of changing its signature.  Most copies are #if 0,
since the parameters have changed.

language is only size 4, so I have corrected writing past its end for
ISO_639_LANGUAGE_DESCRIPTOR.

The language strings will be in the AVDictionary AVStream.metadata under "language".

dvbci->comp_page, dvbci->anc_page, and dvbci->sub_id will become
AVStream->codecpar->extradata: indices mod 5
subtitling_type: 0
comp_page:       RB16(1)
anc_page:        RB16(3)
sub_id:          RB32(1)

dvbci->component_tag will become
(st->stream_identifier - 1) [0 is used for unknown]