core: move initialization of -.slice and init.scope into the unit_load() callbacks

core: move initialization of -.slice and init.scope into the unit_load() callbacks

Previously, we'd synthesize the root slice unit and the init scope unit in the
enumerator callbacks for the unit type. This is problematic if either of them
is already referenced from a unit that is loaded as result of another unit
type's enumerator logic.

Let's clean this up and simply create the two objects from the enumerator
callbacks, if they are not around yet. Do the actual filling in of the settings
from the unit_load() callbacks, to match how other units are loaded.

Fixes: #4322