MDNS: fix legacy unicast responses

[mcspr]

When trying to resolve mdns hostname using lwip built-in resolver (WiFi.hostByName("something.local")), LeaMDNS responder does not construct a response that lwip expects. While it does preserve the request ID here:

Arduino/libraries/ESP8266mDNS/src/LEAmDNS_Control.cpp

Lines 248 to 250 in 48ca7cf

	DEBUG_EX_RX(DEBUG_OUTPUT.printf_P(PSTR("[MDNSResponder] _parseQuery: Legacy query from local host %s!\n"), IPAddress(m_pUDPContext->getRemoteAddress()).toString().c_str()););
	sendParameter.m_u16ID = p_MsgHeader.m_u16ID;

This ID is never set when preparing the message:

[MDNSResponder] _parseQuery: Legacy query from local host 10.0.0.22, id 30!
...
[MDNSResponder] _prepareMDNSMessage: ID:0 QR:1 OP:0 AA:1 TC:0 RD:0 RA:0 R:0 QD:1 AN:1 NS:0 AR:0
...

(debugging message could be improved though)

[mcspr]

For posterity's sake, small example with dig failing:

#include <Arduino.h>
#include <ESP8266WiFi.h>
#include <ESP8266mDNS.h>

#define HOSTNAME "something"

#ifndef USE_SETTINGS
#define WIFI_SSID "SSID"
#define WIFI_PASSWORD "PASSWORD"
#else
#include "settings.h"
#endif

void setup() {

    Serial.begin(115200);
    Serial.println();
    Serial.println();

    delay(1000);

    Serial.printf("> MDNS test\n");
    Serial.printf("> hostname=%s ssid=%s password=%s\n",
        HOSTNAME, WIFI_SSID, WIFI_PASSWORD
    );

    WiFi.persistent(false);
    WiFi.mode(WIFI_STA);
    WiFi.begin(WIFI_SSID, WIFI_PASSWORD);
    while (WiFi.status() != WL_CONNECTED) {
        delay(500);
        Serial.print(".");
    }

    Serial.println();
    Serial.println("> connected")

    MDNS.begin(HOSTNAME);

}

void loop() {
    MDNS.update();
}

$ dig -p 5353 @224.0.0.251 something.local
;; Warning: ID mismatch: expected ID 12327, got 0

[devyte]

code wise this looks ok