Make ipn.Options.Prefs a pointer.

This is a prelude to making it truly optional, once state
management has moved into the backend. For now though, it's
still required. This change is just isolating the bubbling-up
of the pointerification into other layers.

Signed-Off-By: David Anderson <dave@natulte.net>

ipn/fake.go

@@ -21,7 +21,7 @@ func (b *FakeBackend) Start(opts Options) error {
 		log.Fatalf("FakeBackend.Start: opts.Notify is nil\n")
 	}
 	b.notify = opts.Notify
-	b.notify(Notify{Prefs: &opts.Prefs})
+	b.notify(Notify{Prefs: opts.Prefs})
 	nl := NeedsLogin
 	b.notify(Notify{State: &nl})
 	return nil