X-Sendfile support for web response. to use, the webresponse factory needs parameters. set 'use_sendfile_header' to true, and optionally set 'sendfile_header_name' to sth like 'X-LIGHTTPD-send-file' if your webserver doesn't use X-Sendfile. then, plainfile streams in the response will be sent using that instead of fpasshtru, which typically means performance gains of up to 50%, and even more with concurrent requests. closes #431, which also has some links on the topic.