Adding support for username in mqtt

mqtt.c

/* Add the username flag to the CONNECT Variable Header*/
static char user_flag(char flag)
{
    char usr_flag = (char)0x80;
    flag |= usr_flag;
    return flag;
}

/* mqtt_connect_user send the CONNECT command with the 
 * user supplied*/
static CURLcode mqtt_connect_user(struct Curl_easy *data,
        char *pkt, const size_t pktlen)
{
  CURLcode result = CURLE_OK;
  /*magic number that need to be set properly*/
  const size_t client_id_offset = 14;

  /*extracting username from request*/
  const char *username = data->state.aptr.user ?
      data->state.aptr.user : "NOOOOP";
  const size_t ulen = strlen(username);

  const size_t packetlen = pktlen + ulen + 2;

  /*size of all packet*/
  pkt[1] = (packetlen - 2) & 0x7f;
  /*enabling username flag*/
  pkt[9] = user_flag(pkt[9]);
  /*length of username provided*/
  pkt[pktlen + 1] = (char)ulen;

  /*copying username into packet*/
  memcpy(&pkt[client_id_offset + MQTT_CLIENTID_LEN + 2], username, ulen);
  /*sending data*/
  result = mqtt_send(data, pkt, packetlen);
  /*cleaning the user*/
  Curl_safefree(data->state.aptr.user);
  return result;
}

static CURLcode mqtt_connect(struct Curl_easy *data)
{
  CURLcode result = CURLE_OK;
  const size_t client_id_offset = 14;
  char client_id[MQTT_CLIENTID_LEN + 1] = "curl";
  const size_t clen = strlen("curl");

  const size_t packetlen = client_id_offset + MQTT_CLIENTID_LEN;
  char packet[64] = {
    MQTT_MSG_CONNECT,  /* packet type */
    0x00,              /* remaining length */
    0x00, 0x04,        /* protocol length */
    'M','Q','T','T',   /* protocol name */
    0x04,              /* protocol level */
    0x02,              /* CONNECT flag: CleanSession */
    0x00, 0x3c,        /* keep-alive 0 = disabled */
    0x00, 0x00         /* payload1 length */
  };

  packet[1] = (packetlen - 2) & 0x7f;
  packet[client_id_offset - 1] = MQTT_CLIENTID_LEN;

  result = Curl_rand_hex(data, (unsigned char *)&client_id[clen],
                         MQTT_CLIENTID_LEN - clen + 1);
  memcpy(&packet[client_id_offset], client_id, MQTT_CLIENTID_LEN);
  infof(data, "Using client id '%s'\n", client_id);

  if(data->state.aptr.user)
    result = mqtt_connect_user(data, packet, packetlen);
  else
    if(!result)
      result = mqtt_send(data, packet, packetlen);
  return result;
}