Add notion of "internal request arguments"
The referrer fields that Fluid adds to all forms currently end up in the target request and thus in the form action of the next form.
Request arguments that start with two underscores should be "framework internal".
Request::getArguments() should only return arguments that do not start with two underscores and we should add a non-API method
Request::getReferringRequest() that would build a request object on the fly.