Define a hook for fetching management/status items.

This patch was submitted by Ian Holsman. Greg revised some names, applied
the Apache style, and namespace-prefixed the public symbols. Minor bugfix in
the use of the hook implementation macro.

Submitted by: Ian Holsman <IanH@cnet.com>
Reviewed by: Greg Stein


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@89364 13f79535-47bb-0310-9956-ffa450edef68