core: add new OnSuccess= dependency type

This is similar to OnFailure= but is activated whenever a unit returns
into inactive state successfully.

I was always afraid of adding this, since it effectively allows building
loops and makes our engine Turing complete, but it pretty much already
was it was just hidden.

Given that we have per-unit ratelimits as well as an event loop global
ratelimit I feel safe to add this finally, given it actually is useful.

Fixes: #13386