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>

ipn/e2e_test.go

@@ -168,8 +168,8 @@ func newNode(t *testing.T, prefix string, https *httptest.Server) testNode {
 	}
 	n.Start(Options{
 		FrontendLogID: prefix + "-f",
-		ServerURL:     https.URL,
 		Prefs: &Prefs{
+			ControlURL:       https.URL,
 			RouteAll:         true,
 			AllowSingleHosts: true,
 			CorpDNS:          true,