ipn: add DefaultControlURL const, replace few literals with it

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/up.go

@@ -57,7 +57,7 @@ var upFlagSet = (func() *flag.FlagSet {
 	upf.BoolVar(&upArgs.forceReauth, "force-reauth", false, "force reauthentication")
 	upf.BoolVar(&upArgs.reset, "reset", false, "reset unspecified settings to their default values")
 
-	upf.StringVar(&upArgs.server, "login-server", "https://login.tailscale.com", "base URL of control server")
+	upf.StringVar(&upArgs.server, "login-server", ipn.DefaultControlURL, "base URL of control server")
 	upf.BoolVar(&upArgs.acceptRoutes, "accept-routes", false, "accept routes advertised by other Tailscale nodes")
 	upf.BoolVar(&upArgs.acceptDNS, "accept-dns", true, "accept DNS configuration from the admin panel")
 	upf.BoolVar(&upArgs.singleRoutes, "host-routes", true, "install host routes to other Tailscale nodes")

cmd/tailscale/cli/web.go

@@ -246,7 +246,7 @@ func webHandler(w http.ResponseWriter, r *http.Request) {
 // TODO(crawshaw): some of this is very similar to the code in 'tailscale up', can we share anything?
 func tailscaleUp(ctx context.Context) (authURL string, retErr error) {
 	prefs := ipn.NewPrefs()
-	prefs.ControlURL = "https://login.tailscale.com"
+	prefs.ControlURL = ipn.DefaultControlURL
 	prefs.WantRunning = true
 	prefs.CorpDNS = true
 	prefs.AllowSingleHosts = true