Added ceiling on file size for memory mapped files.

Submitted by: John Giannandrea <jg@meer.net>
Reviewed by: Ken, Dean, Randy, Lars, Jim, Ralf
PR: 4122


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@83316 13f79535-47bb-0310-9956-ffa450edef68