AVOptions: deprecate unused AV_OPT_FLAG_METADATA

It was never used since it was added and the things it was intended for
are now exported differently.
AVOptions: add flags for read/read-only options
libavformat/mpegts: expose raw packet size

We cannot easily determine if an mpeg TS's packet size is DVHS, FEC
or so on, for that we need to expose the internal raw_packet_size
field.

Signed-off-by: Anton Khirnov <anton@khirnov.net>