core: use our usual UINT32_MAX scaling for OOMD limits

So far OOMD limits used permyriads, as an upgrade from the original
percent.

The rest of our codebase typically scales stuff relative to UINT32_MAX.
Let's clean this up, an make sure this happens here too. This is
particularly relevant, as this is exposed in unit files and API, and
before we mark this stable we should get the APIs right.