core: Add dirwalk_stat hook.

trunk patch: http://svn.apache.org/r1388447
Submitted by: trawick
Reviewed by: minfrin, jim, sf


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1491476 13f79535-47bb-0310-9956-ffa450edef68