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.c

@@ -178,6 +178,7 @@ static const char* const service_state_table[_SERVICE_STATE_MAX] = {
         [SERVICE_FINAL_SIGKILL] = "final-sigkill",
         [SERVICE_FAILED] = "failed",
         [SERVICE_AUTO_RESTART] = "auto-restart",
+        [SERVICE_CLEANING] = "cleaning",
 };
 
 DEFINE_STRING_TABLE_LOOKUP(service_state, ServiceState);

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;