loadfile: discard prefetched files if demuxer options changed

When using --prefetch-playlist, if demuxer options are changed in the
time window between the start of prefetching and the playback of the
next file, the old values are used. This includes setting demuxer
options in legacy extension auto profiles.

Fix this by setting a flag when demuxer options change and not using the
prefetched data when that flag is true.

UPDATE_DEMUXER is not added to demux.c's options because those already
support updates while playing.