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.

src/core/unit-dependency-atom.c

@@ -85,6 +85,7 @@ static const UnitDependencyAtom atom_map[_UNIT_DEPENDENCY_MAX] = {
          * things discoverable/debuggable as they are the inverse dependencies to some of the above. As they
          * have no effect of their own, they all map to no atoms at all, i.e. the value 0. */
         [UNIT_RELOAD_PROPAGATED_FROM] = 0,
+        [UNIT_ON_FAILURE_OF]          = 0,
 };
 
 UnitDependencyAtom unit_dependency_to_atom(UnitDependency d) {