FileUpload step crashes if file already exists at server (Windows)

[parshin]

If build uploads file to server and the file already exists on server, step crashes with WindowsError:

Traceback (most recent call last):
	  File "c:Python26libsite-packages	wistedspreadanana.py", line 153, in gotItem
	    self.callExpressionReceived(item)
	  File "c:Python26libsite-packages	wistedspreadanana.py", line 116, in callExpressionReceived
	    self.expressionReceived(obj)
	  File "c:Python26libsite-packages	wistedspreadpb.py", line 514, in expressionReceived
	    method(*sexp[1:])
	  File "c:Python26libsite-packages	wistedspreadpb.py", line 826, in proto_message
	    self._recvMessage(self.localObjectForID, requestID, objectID, message, answerRequired, netArgs, netKw)
	--- <exception caught here> ---
	  File "c:Python26libsite-packages	wistedspreadpb.py", line 840, in _recvMessage
	    netResult = object.remoteMessageReceived(self, message, netArgs, netKw)
	  File "c:Python26libsite-packages	wistedspreadflavors.py", line 114, in remoteMessageReceived
	    state = method(*args, **kw)
	  File "c:Python26libsite-packagesuildbotsteps	ransfer.py", line 58, in remote_close
	    os.rename(self.tmpname, self.destfile)
	exceptions.WindowsError: [Error 183] Cannot create a file when that file already exists

Maybe, file on server should be deleted before rename (or when step starts?)...

[Dustin J. Mitchell]

Unlink the destination for an upload before renaming

This is required on Windows, which does not implicitly unlink the destination in rename. Tested by hand. Fixes #1058.

Changeset: 1b69b024b28bf29e204a6a4d38f8ead642e819b8