core: don't dispatch load queue when setting Slice= for transient units

core: don't dispatch load queue when setting Slice= for transient units

Let's be more careful when setting up the Slice= property of transient units:
let's use manager_load_unit_prepare() instead of manager_load_unit(), so that
the load queue isn't dispatched right away, because our own transient unit is
in it, and we don#t want to have it loaded until we finished initializing it.