core: make sure targets that get a default Conflicts=shutdown.target are also ordered...

core: make sure targets that get a default Conflicts=shutdown.target are also ordered against it

Let's tweak the automatic dependency generation of target units: not only add a
Conflicts= towards shutdown.target but also an After= line for it, so that we
can be sure the new target is not started when the old target is still up.

Discovered in the context of #4733

(Also, exclude dependency generation if for shutdown.target itself. — This is
strictly speaking redundant, as unit_add_two_dependencies_by_name() detects
that and becomes a NOP, but let's make this explicit for readability.)