Add 'type' parameter to net.Server.listenFD()

This is needed in case the provided socket is not the default 'tcp4' type
(i.e. and needs different read/write/etc methods). With this patch, one can
call listenFD(sock, 'unix') to bind to existing UNIX domain sockets.

lib/net.js

@@ -1167,12 +1167,13 @@ Server.prototype.listen = function () {
   }
 };
 
-Server.prototype.listenFD = function (fd) {
+Server.prototype.listenFD = function (fd, type) {
   if (this.fd) {
     throw new Error('Server already opened');
   }
 
   this.fd = fd;
+  this.type = type || null;
   this._startWatcher();
 };