This introduces a new request field send_content which indicates if the
content should be sent.  This covers various cases like HEAD responses
where there's an entity, but no content is sent as well as 204, 304, and 1xx
responses where there is not even an entity.

writebuf() will now enforce this, so there's no need for various checks
throughout the code to see if data should be sent or not.

Also, we now only set sent_headers after the headers are sent.