core: turn unit_load_fragment_and_dropin_optional() into a flag

core: turn unit_load_fragment_and_dropin_optional() into a flag

unit_load_fragment_and_dropin() and unit_load_fragment_and_dropin_optional()
are really the same, with one minor difference in behaviour. Let's drop
the second function.

"_optional" in the name suggests that it's the "dropin" part that is optional.
(Which it is, but in this case, we mean the fragment to be optional.)
I think the new version with a flag is easier to understand.