path: Close inotify FD asynchronously

path: Close inotify FD asynchronously

inotify FD may take several milliseconds to close.  We measured
daemon-reload

        default: (0.427 ± 0.05) s
        async:   (0.323 ± 0.02) s

with 5 path units out of 422 units. I.e. ~1% of units cause ~25% of
delay, hence this fix seems like low-hanging fruit on the daemon-reload
critical path.

Particular inotify slowness pointed out by @fbuihuu.