ipn: move Options.ServerURL into Prefs.

We can't rely on a frontend to provide a control
server URL, so this naturally belongs in server-persisted
state.

Signed-off-by: David Anderson <dave@natulte.net>

cmd/tailscale/tailscale.go

@@ -78,6 +78,7 @@ func main() {
 	// TODO(apenwarr): fix different semantics between prefs and uflags
 	// TODO(apenwarr): allow setting/using CorpDNS
 	prefs := ipn.Prefs{
+		ControlURL:       *server,
 		WantRunning:      true,
 		RouteAll:         *routeall,
 		AllowSingleHosts: !*nuroutes,
@@ -106,8 +107,7 @@ func main() {
 	bc := ipn.NewBackendClient(log.Printf, clientToServer)
 	bc.SetPrefs(prefs)
 	opts := ipn.Options{
-		StateKey:  globalStateKey,
-		ServerURL: *server,
+		StateKey: globalStateKey,
 		Notify: func(n ipn.Notify) {
 			if n.ErrMessage != nil {
 				log.Fatalf("backend error: %v\n", *n.ErrMessage)