Better logic for testing if an argument is a port

If you did server.listen('123') it would open a socket in the current
directory called 123. Now it will interpret it as a port.

lib/net.js

@@ -759,6 +759,8 @@ function doConnect (socket, port, host) {
   };
 }
 
+function isPort (x) { return parseInt(x) >= 0; }
+
 
 // var stream = new Stream();
 // stream.connect(80)               - TCP connect to port 80 on the localhost
@@ -774,7 +776,7 @@ Stream.prototype.connect = function () {
 
   self._connecting = true; // set false in doConnect
 
-  if (parseInt(arguments[0]) >= 0) {
+  if (isPort(arguments[0])) {
     // TCP
     var port = arguments[0];
     dns.lookup(arguments[1], function (err, ip, addressType) {
@@ -991,7 +993,7 @@ Server.prototype.listen = function () {
   var self = this;
   if (self.fd) throw new Error('Server already opened');
 
-  if (typeof(arguments[0]) == 'string') {
+  if (!isPort(arguments[0])) {
     // the first argument specifies a path
     self.fd = socket('unix');
     self.type = 'unix';