core: when adding names to unit, require matching instance strings

core: when adding names to unit, require matching instance strings

We would check that the instance is present in both units (or missing in both).
But when it is defined, it should be the same in both. The comment in the code
was explicitly saying that differing instance strings are allowed, but this
mostly seems to be a left-over from old times. The man page is pretty clear:

> the instance (if any) is always uniquely defined for a given unit and all its
> aliases.