FastCGI: protection from responses with wrong length.

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.