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/message_test.go

@@ -96,6 +96,7 @@ func TestClientServer(t *testing.T) {
 	ch := make(chan Notify, 256)
 	h, err := NewHandle(bc, clogf, Options{
 		ServerURL: "http://example.com/fake",
+		Prefs:     &Prefs{},
 		Notify: func(n Notify) {
 			ch <- n
 		},