core: make use of unit_active_or_pending() where we can

core: make use of unit_active_or_pending() where we can

Let's make use of unit_active_or_pending() where we can. Note that this
change changes beaviour in one specific case: when shutdown.target is
active we'll now also return that the system is in "stopping" state, not
only when we try to get into it. That makes sense as shutdown.target is
ordered before the actually shutdown units such as
"systemd-poweroff.service", and if the state is queried between reaching
those we should also report "stopping".