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-engine.c

@@ -93,7 +93,7 @@ int main(int argc, char *argv[]) {
         assert_se(set_unit_path(unit_dir) >= 0);
         assert_se(runtime_dir = setup_fake_runtime_dir());
 
-        r = manager_new(LOOKUP_SCOPE_USER, MANAGER_TEST_RUN_BASIC, &m);
+        r = manager_new(RUNTIME_SCOPE_USER, MANAGER_TEST_RUN_BASIC, &m);
         if (manager_errno_skip_test(r))
                 return log_tests_skipped_errno(r, "manager_new");
         assert_se(r >= 0);