demux_lavf: fix timebase confusion

This set last_pts to bogus values, which is used for relative seeks.
The player usually uses absolute seeks, so this didn't matter much.
dec_video: fix handling of timestamp resets

This code tried to pass a still monotonic (even if not strictly
monotonic) PTS to the player, but as a result it remained stuck on
the PTS before a reset (since the PTS was lower).
dvdnav: support mouse interaction
build: dvdnav needs dvdread
dvb: prevent seek when reopening the demuxer

Since dvb input is not seekable anyway, this would probably just flush
the stream buffer, but at least it'll avoid a bogus message about
seeking errors.