api: check unit name on set operations

since we essentially have two fields where the unit name can be set I think it's safest to take this path and ensure they're the same. I also don't feel strongly about the auto-fill so can remove that if you don't like it.

validateName is dumb right now but anticipating #867....
Yes, and how about we clarify what "item name" means since it isn't exactly clear to an end-user. How about this: "name in URL %q differs from name in request body %q"