API: new ap_custom_response() function for hooking into the

     ErrorDocument mechanism at runtime
PR:
Obtained from:
Submitted by:	Doug MacEachern
Reviewed by:	Ralf


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@81875 13f79535-47bb-0310-9956-ffa450edef68