core/mount: add dependencies to dynamically mounted mounts too

core/mount: add dependencies to dynamically mounted mounts too

Add unit dependencies for dynamic (i. e. not from fstab) mounts. With that,
mount units properly bind to their underlying device, and thus get
automatically stopped/unmounted when the underlying device goes away.

This cleans up stale mounts from unplugged devices.

Thanks to Lennart Poettering for pointing out the fix!