internal/router: add UniFi Gateway support

UniFi Gateway (USG) uses its own DNS forwarding rule, which is
configured default in /etc/dnsmasq.conf file. Adding ctrld own config in
/etc/dnsmasq.d won't take effects. Instead, we must make changes
directly to /etc/dnsmasq.conf, configuring ctrld as the only upstream.

internal/router/router.go

@@ -220,8 +220,10 @@ func distroName() string {
 	case bytes.HasPrefix(unameO(), []byte("Tomato")):
 		return Tomato
 	case haveDir("/config/scripts/post-config.d"):
+		checkUSG()
 		return EdgeOS
 	case haveFile("/etc/ubnt/init/vyatta-router"):
+		checkUSG()
 		return EdgeOS // For 2.x
 	case isPfsense():
 		return Pfsense
@@ -253,3 +255,8 @@ func isPfsense() bool {
 	b, err := os.ReadFile("/etc/platform")
 	return err == nil && bytes.HasPrefix(b, []byte("pfSense"))
 }
+
+func checkUSG() {
+	out, _ := exec.Command("mca-cli-op", "info").Output()
+	isUSG = bytes.Contains(out, []byte("UniFi-Gateway-"))
+}