core: explicitly trigger changing udev SYSTEMD_WANTS properties

core: explicitly trigger changing udev SYSTEMD_WANTS properties

This compensates for the unsynchronized reload cycles of systemd and
udev: we manually trigger the deps listed in SYSTEMD_WANTS properties if
they change for device units that are already up. That way all deps
defined that way will be triggered at least once: the first time the
unit goes up by the usual dependency logic, and if it already is up by
the device.c specific logic.

Fixes: #9323