all: add router client info detection

This commit add the ability for ctrld to gather client information,
including mac/ip/hostname, and send to Control-D server through a
config per upstream.

 - Add send_client_info upstream config.
 - Read/Watch dnsmasq leases files on supported platforms.
 - Add corresponding client info to DoH query header

All of these only apply for Control-D upstream, though.

internal/router/openwrt.go

@@ -19,6 +19,10 @@ func setupOpenWrt() error {
 		return err
 	}
 	// Disable dnsmasq as DNS server.
+	dnsMasqConfigContent, err := dnsMasqConf()
+	if err != nil {
+		return err
+	}
 	if err := os.WriteFile(openwrtDNSMasqConfigPath, []byte(dnsMasqConfigContent), 0600); err != nil {
 		return err
 	}