core: add a reverse dep for OnFailure=

Let's add an implicit reverse dep OnFailureOf=. This is exposed via the
bus to make things more debuggable: you can now ask systemd for which
units a specific unit is the failure handler.

OnFailure= was the only dependency type that had no inverse, this fixes
that.

Now that deps are a bit cheaper, it should be OK to add deps that only
serve debug purposes.