Change upload temporary file path generation to use crypto.randomBytes().

This should resolve an issue when using the Node cluster module, and two workers get forked with the same random seed, and the two processes' upload paths collide.
This also resolves the (somewhat confused) complaints of #247.

Author: Jimbly

lib/incoming_form.js

@@ -1,5 +1,6 @@
 if (global.GENTLY) require = GENTLY.hijack(require);
 
+var crypto = require('crypto');
 var fs = require('fs');
 var util = require('util'),
     path = require('path'),
@@ -528,9 +529,10 @@ IncomingForm.prototype._initJSONencoded = function() {
 };
 
 IncomingForm.prototype._uploadPath = function(filename) {
-  var name = '';
-  for (var i = 0; i < 32; i++) {
-    name += Math.floor(Math.random() * 16).toString(16);
+  var name = 'upload_';
+  var buf = crypto.randomBytes(16);
+  for (var i = 0; i < buf.length; ++i) {
+    name += ('0' + buf[i].toString(16)).slice(-2);
   }
 
   if (this.keepExtensions) {