core: move ManagerRunningAs to shared

Note: I did s/MANAGER/SYSTEMD/ everywhere, even though it makes the
patch quite verbose. Nevertheless, keeping MANAGER prefix in some
places, and SYSTEMD prefix in others would just lead to confusion down
the road. Better to rip off the band-aid now.

src/test/test-engine.c

@@ -33,7 +33,7 @@ int main(int argc, char *argv[]) {
 
         assert_se(set_unit_path("test") >= 0);
 
-        assert_se(manager_new(MANAGER_SYSTEM, &m) >= 0);
+        assert_se(manager_new(SYSTEMD_SYSTEM, &m) >= 0);
 
         printf("Load1:\n");
         assert_se(manager_load_unit(m, "a.service", NULL, NULL, &a) >= 0);