Make the old_write filter use the ap_f* functions for buffering the data.

This has been tested with a couple of directory listings, but it could
probably use a bit more testing before being declared stable.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@88454 13f79535-47bb-0310-9956-ffa450edef68