basic: add RuntimeScope enum

In various tools and services we have a per-system and per-user concept.
So far we sometimes used a boolean indicating whether we are in system
mode, or a reversed boolean indicating whether we are in user mode, or
the LookupScope enum used by the lookup path logic.

Let's address that, in introduce a common enum for this, we can use all
across the board.

This is mostly just search/replace, no actual code changes.

src/test/test-socket-bind.c

@@ -132,7 +132,7 @@ int main(int argc, char *argv[]) {
         assert_se(set_unit_path(unit_dir) >= 0);
         assert_se(runtime_dir = setup_fake_runtime_dir());
 
-        assert_se(manager_new(LOOKUP_SCOPE_USER, MANAGER_TEST_RUN_BASIC, &m) >= 0);
+        assert_se(manager_new(RUNTIME_SCOPE_USER, MANAGER_TEST_RUN_BASIC, &m) >= 0);
         assert_se(manager_startup(m, NULL, NULL, NULL) >= 0);
 
         assert_se(test_socket_bind(m, "socket_bind_test.service", netcat_path, "2000", STRV_MAKE("2000"), STRV_MAKE("any")) >= 0);