matroska: add --ordered-chapters-files option

This option takes a playlist. The playlist will then be used as list of
potential segment files for use with ordered chapters.
demux_mkv: don't seek outside of the file when finding segments

The end of the current segment will be the end of the file if there is
no next segment. Normally, this didn't matter much, since UNIX files
allow seeking past the end of the file. But when opening files from
HTTP, this would print confusing error messages. So explicitly check for
EOF before trying to read a segment.