internal/router: make router.Cleanup idempotent

On routers where we want to wait for NTP by checking nvram key. Before
waiting, we clean up the router to ensure it's restored to original
state. However, router.Cleanup is not idempotent, causing dnsmasq
restarted. On tomato/ddwrt, restarting have no delay, and spawning new
dnsmasq process immediately. On merlin, somehow it takes time to spawn
new dnsmasq process, causing ctrld wrongly think there's no one
listening on port 53.

Fixing this by ensuring router.Cleanup is idempotent. While at it, also
adding "ntp_done" to nvram key, which is now using on latest ddwrt.

internal/router/tomato/tomato.go

@@ -89,12 +89,14 @@ func (f *FreshTomato) Cleanup() error {
 	if f.cfg.FirstListener().IsDirectDnsListener() {
 		return nil
 	}
-	if val, _ := nvram.Run("get", nvram.CtrldSetupKey); val == "1" {
-		nvramKvMap["dnsmasq_custom"] = ""
-		// Restore old configs.
-		if err := nvram.Restore(nvramKvMap, nvram.CtrldSetupKey); err != nil {
-			return err
-		}
+	if val, _ := nvram.Run("get", nvram.CtrldSetupKey); val != "1" {
+		return nil // was restored, nothing to do.
+	}
+
+	nvramKvMap["dnsmasq_custom"] = ""
+	// Restore old configs.
+	if err := nvram.Restore(nvramKvMap, nvram.CtrldSetupKey); err != nil {
+		return err
 	}
 
 	// Restart dnscrypt-proxy service.