core: reorder where we add units to queues in unit_notify()

This moves all calls that shall do deferred work on detecting whether to
start/stop the unit or dependent units after a unit state change to the
end of the function, to make things easier to read.

So far, these calls were spread all over the function, and
conditionalized needlessly on MANAGER_RELOADING(). This is unnecessary,
since the queues are not dispatched while reloading anyway, and
immediately before acting on a queued unit we'll check if the suggested
operation really makes sense.

The only conditionalizaiton we leave in is on checking the new unit
state itself, since we have that in a local variable anyway.