bus: introduce concept of a "default" event loop per-thread and make use of it everywhere

Try to emphasize a bit that there should be a mapping between event
loops and threads, hence introduce a logic that there's one "default"
event loop for each thread, that can be queried via
"sd_event_default()".

src/machine/machined.c

@@ -53,7 +53,7 @@ Manager *manager_new(void) {
                 return NULL;
         }
 
-        r = sd_event_new(&m->event);
+        r = sd_event_default(&m->event);
         if (r < 0) {
                 manager_free(m);
                 return NULL;