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.c

@@ -2871,7 +2871,8 @@ int unit_add_dependency(
                 [UNIT_CONFLICTED_BY] = UNIT_CONFLICTS,
                 [UNIT_BEFORE] = UNIT_AFTER,
                 [UNIT_AFTER] = UNIT_BEFORE,
-                [UNIT_ON_FAILURE] = _UNIT_DEPENDENCY_INVALID,
+                [UNIT_ON_FAILURE] = UNIT_ON_FAILURE_OF,
+                [UNIT_ON_FAILURE_OF] = UNIT_ON_FAILURE,
                 [UNIT_REFERENCES] = UNIT_REFERENCED_BY,
                 [UNIT_REFERENCED_BY] = UNIT_REFERENCES,
                 [UNIT_TRIGGERS] = UNIT_TRIGGERED_BY,