Added extended attribute 'where from' logging for downloads

On OS X it is possible to get list of extended attributes of a file. For some file (e.g. downloads) these lists contain *where from?* information indicating website that the file came from originally.
Why "kd_" prefix? I would be in favor of something simple like "where_from". Otherwise if we want to keep canonical name of extended attribute name it would be something like "kmd_where_from" or "kmd_item_where_from": they are both quite lengthy.