Parsing issues with mDNS packets

[francais01]

Multicast DNS packets have at least 2 characteristics which packet.js does not deal with:

• A "reply" will not necessarily have a question, just answer RRs.
  I worked around this by adding an escape at the beginning of parseQuestion():

if (packet.header.opcode === 0 && // Standard query
  packet.header.qr === 1 && // Response
  packet.question.length === 0) { // Empty question can occur in mDNS
  return PARSE_RESOURCE_RECORD;
}

• Some mDNS packets flying around on the network have multiple questions.
  I hacked up a quick loop in parseQuestion() and removed the assert:

var qnum = packet.question.length;
while (qnum > 0) {
  var val = {};
  val.name = nameUnpack(msg);
  val.type = msg.readUInt16BE();
  val.class = msg.readUInt16BE();
  packet.question[packet.question.length - qnum] = val;
  qnum -= 1;
}