Changes and fixes

• all: make BufferedWatcher buffered again (#657)

• inotify: fix race when adding/removing watches while a watched path is being deleted (#678, #686)

• inotify: don't send empty event if a watched path is unmounted (#655)

• inotify: don't register duplicate watches when watching both a symlink and its target; previously that would get "half-added" and removing the second would panic (#679)

• kqueue: fix watching relative symlinks (#681)

• kqueue: correctly mark pre-existing entries when watching a link to a dir on kqueue (#682)

• illumos: don't send error if changed file is deleted while processing the event (#678)