core: simplify the return convention in manager_load_unit()

core: simplify the return convention in manager_load_unit()

This function was returning 0 or 1 on success. It has many callers, and it
wasn't clear if any of them care about the distinction. It turns out they don't
and the return values were done for convenience because manager_load_unit_prepare()
returns 0 or 1. Let's invert the code in the static function to follow the usual
pattern where 0 means "no work was done" and 1 means "work was done", and make
the non-static function always return 0 to make the code easier to read, and
also add comments that explain what is happening.

No functional change.