FastCGI: protection from responses with wrong length. Previous behaviour was to pass everything to the client, but this seems to be suboptimal and causes issues (ticket #1695). Fix is to drop extra data instead, as it naturally happens in most clients. Additionally, we now also issue a warning if the response is too short, and make sure the fact it is truncated is propagated to the client. The u->error flag is introduced to make it possible to propagate the error to the client in case of unbuffered proxying. For responses to HEAD requests there is an exception: we do allow both responses without body and responses with body matching the Content-Length header.