cache in the event that the client cancels the transfer, provided that

the configured percentage of the file has already been transfered.
It works for http transfers only.

The new httpd.conf directive is:
CacheForceCompletion <percentage>

PR: 2277
Submitted by: 	Glen Parker <glenebob@nwlink.com>
Reviewed by:	Martin Kraemer, Brian Behlendorf


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@81685 13f79535-47bb-0310-9956-ffa450edef68