Pass through the seek flags to the stream layer. The STREAM_CTRL
semantics become a bit awkward, but that's still the least awkward
part about optical disc media.
Make demux_disc.c request relative seeks. Now the player will use
relative seeks if the user sends relative seek commands, and the
demuxer announces it wants these by setting rel_seeks to true. This
change probably changes seek behavior for dvd, dvdnav, bluray, cdda,
and possibly makes seeking useless if the demuxer-cache is set to
a high value.
Will be used in the next commit. (Split to make reverting the next
commit easier.)