ipn: move Options.Notify to its own method

We already had SetNotifyCallback elsewhere on controlclient, so use
that name.

Baby steps towards some CLI refactor work.

Updates tailscale/tailscale#1436

ipn/message_test.go

@@ -90,13 +90,11 @@ func TestClientServer(t *testing.T) {
 	bc = NewBackendClient(clogf, clientToServer)
 
 	ch := make(chan Notify, 256)
-	h, err := NewHandle(bc, clogf, Options{
+	notify := func(n Notify) { ch <- n }
+	h, err := NewHandle(bc, clogf, notify, Options{
 		Prefs: &Prefs{
 			ControlURL: "http://example.com/fake",
 		},
-		Notify: func(n Notify) {
-			ch <- n
-		},
 	})
 	if err != nil {
 		t.Fatalf("NewHandle error: %v\n", err)