Simplified sendfile_max_chunk handling. Previously, it was checked that sendfile_max_chunk was enabled and almost whole sendfile_max_chunk was sent (see e67ef50c3176), to avoid delaying connections where sendfile_max_chunk wasn't reached (for example, when sending responses smaller than sendfile_max_chunk). Now we instead check if there are unsent data, and the connection is still ready for writing. Additionally we also check c->write->delayed to ignore connections already delayed by limit_rate. This approach is believed to be more robust, and correctly handles not only sendfile_max_chunk, but also internal limits of c->send_chain(), such as sendfile() maximum supported length (ticket #1870).

scgi_param  REQUEST_METHOD     $request_method;
scgi_param  REQUEST_URI        $request_uri;
scgi_param  QUERY_STRING       $query_string;
scgi_param  CONTENT_TYPE       $content_type;

scgi_param  DOCUMENT_URI       $document_uri;
scgi_param  DOCUMENT_ROOT      $document_root;
scgi_param  SCGI               1;
scgi_param  SERVER_PROTOCOL    $server_protocol;
scgi_param  REQUEST_SCHEME     $scheme;
scgi_param  HTTPS              $https if_not_empty;

scgi_param  REMOTE_ADDR        $remote_addr;
scgi_param  REMOTE_PORT        $remote_port;
scgi_param  SERVER_PORT        $server_port;
scgi_param  SERVER_NAME        $server_name;