mpegts-mythtv: harmonize pmt_cb() part 7

ff_parse_mpeg2_descriptor() is now minimally modified. Yay!
This has enabled removing the struct dvb_caption_info_t,
which was only a shim for copying the values from
ff_parse_mpeg2_descriptor() to the AVStream in mpegts_add_stream().

struct pmt_entry_t is now moved out of the header since it is no
longer used there.

mpegts-mythtv.h is now (ignoring the renamed functions) a superset
of mpegts.h.

PMT_PIDS_MAX was 256, MAX_STREAMS_PER_PROGRAM is 128.
I don't anticipate this being an issue, and if it is, it is an
upstream issue.

mpegts_add_stream() has been merged into pmt_cb() with upstream additions.

The streams_changed callback has been disabled as it is now unnecessary
in my samples from the Finnish YLE that used to trigger it.  It could
be re-enabled if necessary.

The following now unused functions have been removed:
mpegts_find_stream_type_pmt
av_mpegts_remove_stream
mpegts_add_stream

This needs testing with DSMCC/MHEG streams.