unit: When stopping due to BindsTo=, log which unit caused it

I'm trying to track down a relatively recent change in systemd
which broke OSTree; see https://bugzilla.gnome.org/show_bug.cgi?id=743891

Systemd started to stop sysroot.mount, and this patch should help
me debug why at least.

While we're here, "break" on the first unit we find that will
deactivate, as there's no point in further iteration.