fs.link(srcpath, dstpath, [callback(err)])

fs.link = function(srcpath, dstpath, callback) {
  callback = makeCallback(callback);
  if (!nullCheck(srcpath, callback)) return;
  if (!nullCheck(dstpath, callback)) return;
  binding.link(pathModule._makeLong(srcpath),
               pathModule._makeLong(dstpath),
               callback);
};