core: introduce a new job mode JOB_RESTART_DEPENDENCIES

core: introduce a new job mode JOB_RESTART_DEPENDENCIES

This new job mode will enqueue a start job for a unit, and all units
depending on the unit will get a restart job enqueued. This is then used
for automatic sevice restarts: the unit itself is only started, the
depending units restarted. This way the unit will not go down
unnecessarily, triggering OnSuccess= needlessly.

This also introduces a new state SERVICE_AUTO_RESTART_QUEUED that is
entered once the restart jobs are enqueued. Previously we'd stay in
SERVICE_AUTO_RESTART, but that's problematic, since we'd lose
information whether we still need to enqueue the restart job during a
serialization/deserialization cycle or not. By having an explicit state
for this we know exactly whether we still need to enqueue the job or
not. It's also good since when we are in SERVICE_AUTO_RESTART_QUEUED we
want to act on unit_start(), but on SERVICE_AUTO_RESTART we want to wait
for the holdoff time to pass before we act on unit_start().

Fixes: #27722