lavf: call the new seek API from the old.

If the demuxer implements read_seek2() and not read_seek(),
call avformat_seek_file() from av_seek_frame().
Allow to properly seek in formats that only implement the
new API from applications that use the old one.
Tested with mplayer and a concat script.
lavf: filter out AVSEEK_FLAG_BACKWARD in new API.

Also document that it has no effect.
lavf/concatdec: remove invalid check for AVSEEK_FLAG_BACKWARD.
lavf/matroskaenc: return an error for unsupported types.
Merge remote-tracking branch 'cigaes/master'

* cigaes/master:
  lavf/matroskaenc: return an error for unsupported types.
  lavf/concatdec: remove invalid check for AVSEEK_FLAG_BACKWARD.
  lavf: filter out AVSEEK_FLAG_BACKWARD in new API.
  lavf: call the new seek API from the old.

Merged-by: Michael Niedermayer <michaelni@gmx.at>