mpegts-mythtv.c: harmonize functions 1e

Now identical to FFmpeg's:
analyze
get_packet_size
mpegts_probe
seek_back

mpegts_read_header (heavy MythTV modification)

needs
read_packet
finished_reading_packet
handle_packets

which required:
mpegts_resync
handle_packet (modified, signature only)
mpegts_raw_read_packet

which required:
avpriv_mpegts_parse_packet