X-Sendfile support for fetch.php

This patch enables the use of the X-Sendfile extension offered by certain
webservers to deliver static files after running a dynamic script. This
combines the flexibility of a PHP file to check for authorization, caching
and resizing with the low memory footprint and high performance of static
file delivery of the webserver.

See http://blog.lighttpd.net/articles/2006/07/02/x-sendfile for details

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