FS#2388 Fix sendfile/X-Accel-Redirect on nginx

This should fix [FS#2388](https://bugs.dokuwiki.org/index.php?do=details&task_id=2388)

Problem was, nginx sendfile requires the relative path from web servers document root.

I've no real setup to test this fix, we have to wait for test from users in bug report.

Additionally i adjusted all http_sendfile calls in core to the function signature.