inotify passthrough support

[ToxicFrog]

Changes to the backing filesystem are not propagated to inotify watchers on the ffmpegfs mount, which means that if you have a setup like:

Media server using inotify to pick up changes
      🠇
ffmpegfs
      🠇
actual media files

And you move some files around or drop some new files in, the media server won't pick them up until you have it do a full rescan, which is expensive, especially if some cache entries have expired and ffmpegfs has to re-transcode a bunch of files.