This patch adds support in mpegts muxer for using service_provider and
service_name metadata to set service_provider_name and service_name
fields in SDT.

Example usage:
   ffmpeg -i file.ts -f mpegts -re -acodec copy -vcodec copy -f mpegts \
       -metadata service_provider="Some provider" \
       -metadata service_name="Some Channel" \
       "udp://239.0.70.2:5000?pkt_size=1316&ttl=1"
Signed-off-by: Mans Rullgard <mans@mansr.com>