cmd/ctrld: improving ctrld stability on router

The current state of ctrld is very "high stakes" and easy to mess up,
and is unforgiving when "ctrld start" failed. That would cause the
router is in broken state, unrecoverable.

This commit makes these changes to improve the state:

 - Moving router setup process after ctrld listeners are ready, so
   dnsmasq won't flood requests to ctrld even though the listeners are
   not ready to serve requests.

 - On router, when ctrld stopped, restore router DNS setup. That leaves
   the router in good state on reboot/startup, help removing the custom
   DNS server for NTP synchronization on some routers.

 - If self-check failed, uninstall ctrld to restore router to good
   state, prevent confusion that ctrld process is still running even
   though self-check reports it did not started.

internal/router/router.go

@@ -118,7 +118,7 @@ func PreRun() (err error) {
 	switch Name() {
 	case Merlin, Tomato:
 		// Wait until `ntp_ready=1` set.
-		b := backoff.NewBackoff("PreStart", func(format string, args ...any) {}, 10*time.Second)
+		b := backoff.NewBackoff("PreRun", func(format string, args ...any) {}, 10*time.Second)
 		for {
 			out, err := nvram("get", "ntp_ready")
 			if err != nil {