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.

headers/private/kernel/Notifications.h

@@ -152,6 +152,9 @@ public:
 
 	virtual const char*			Name() { return fName; }
 
+			status_t			Register();
+			void				Unregister();
+
 protected:
 	virtual status_t			ToEventMask(const KMessage& eventSpecifier,
 									uint32& eventMask);