Add "watch" mode to reload playlist on changes.

The default implementation, based on mtime, works well, but the inotify
implementation is not working yet...
Working implementation using Duppy!
Add file.watch to register a handler on file modif
Add reload_mode="watch" to playlist.once.
Expand ~ in file.watch.
Implement unwatch in mtime.
Correctly (un)register watchdog for playlists.
Make inotify stuff dynloadable
Return unwatch function in file.watch
Beautification
Mutexify..
Fix.
Add source.on_shutdown.
Make inotify watcher build again...

It's not working anymore though :(...
Move to the right place.
Simplify configure.ml

I moved away types in File_watcher in order to simplify configure (and
make possible modification of file_watcher without reconfiguring). Also,
we now watch for a list of events (which can only contain `Modify for
now...)
Add license headers.
Rename.
Merge branch 'master' into inotify
Put more stuff in critical section..