manager: introduce MANAGER_IS_FINISHED() macro

Let's make our finished checks a bit more readable. Checking the
timestamp is not entirely obvious, hence let's abstract that a bit by
adding a macro that shows what we are doing here, not how we doing it.

This is particularly useful if we want to change the definition of
"finished" later on, in particular, when we try to fix #7023.

src/core/manager.c

@@ -3175,7 +3175,7 @@ void manager_check_finished(Manager *m) {
         /* This is no longer the first boot */
         manager_set_first_boot(m, false);
 
-        if (dual_timestamp_is_set(m->timestamps + MANAGER_TIMESTAMP_FINISH))
+        if (MANAGER_IS_FINISHED(m))
                 return;
 
         dual_timestamp_get(m->timestamps + MANAGER_TIMESTAMP_FINISH);
@@ -3533,7 +3533,7 @@ ManagerState manager_state(Manager *m) {
         assert(m);
 
         /* Did we ever finish booting? If not then we are still starting up */
-        if (!dual_timestamp_is_set(m->timestamps + MANAGER_TIMESTAMP_FINISH)) {
+        if (!MANAGER_IS_FINISHED(m)) {
 
                 u = manager_get_unit(m, SPECIAL_BASIC_TARGET);
                 if (!u || !UNIT_IS_ACTIVE_OR_RELOADING(unit_active_state(u)))