core: hook up service unit type with the new clean operation

The implementation is pretty straight-foward: when we get a request to
clean some type of resources we fork off a process doing that, and while
it is running we are in the "cleaning" state.

src/basic/unit-def.h

@@ -117,6 +117,7 @@ typedef enum ServiceState {
         SERVICE_FINAL_SIGKILL,
         SERVICE_FAILED,
         SERVICE_AUTO_RESTART,
+        SERVICE_CLEANING,
         _SERVICE_STATE_MAX,
         _SERVICE_STATE_INVALID = -1
 } ServiceState;