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-path.c

@@ -32,7 +32,7 @@ static int setup_test(Manager **m) {
         if (r == -ENOMEDIUM)
                 return log_tests_skipped("cgroupfs not available");
 
-        r = manager_new(LOOKUP_SCOPE_USER, MANAGER_TEST_RUN_BASIC, &tmp);
+        r = manager_new(RUNTIME_SCOPE_USER, MANAGER_TEST_RUN_BASIC, &tmp);
         if (manager_errno_skip_test(r))
                 return log_tests_skipped_errno(r, "manager_new");
         assert_se(r >= 0);