Rework DefaultNotificationService registration.

- Instead of implicitly registering and unregistering a service
  instance on construction/destruction, DefaultNotificationService
  now exports explicit Register()/Unregister() calls, which subclasses
  are expected to call when they're ready.

- Adjust all implementing subclasses. Resolves an issue with deadlocks
  when booting a DEBUG=1 build.

src/system/kernel/Notifications.cpp

@@ -156,13 +156,11 @@ DefaultNotificationService::DefaultNotificationService(const char* name)
 	fName(name)
 {
 	recursive_lock_init(&fLock, name);
-	NotificationManager::Manager().RegisterService(*this);
 }
 
 
 DefaultNotificationService::~DefaultNotificationService()
 {
-	NotificationManager::Manager().UnregisterService(*this);
 	recursive_lock_destroy(&fLock);
 }
 
@@ -253,6 +251,20 @@ DefaultNotificationService::RemoveListener(const KMessage* eventSpecifier,
 }
 
 
+status_t
+DefaultNotificationService::Register()
+{
+	return NotificationManager::Manager().RegisterService(*this);
+}
+
+
+void
+DefaultNotificationService::Unregister()
+{
+	NotificationManager::Manager().UnregisterService(*this);
+}
+
+
 status_t
 DefaultNotificationService::ToEventMask(const KMessage& eventSpecifier,
 	uint32& eventMask)