basic/cgroup-util: let cgroup_unified_flush() return the detected hierarchy

This avoid the use of the global variable.

Also rename cgroup_unified_update() to cgroup_unified_cached() and
cgroup_unified_flush() to cgroup_unified() to better reflect their new roles.

src/test/test-condition.c

@@ -124,7 +124,7 @@ static void test_condition_test_control_group_controller(void) {
         _cleanup_free_ char *controller_name = NULL;
         int r;
 
-        r = cg_unified_flush();
+        r = cg_unified();
         if (r < 0) {
                 log_notice_errno(r, "Skipping ConditionControlGroupController tests: %m");
                 return;