core: Add post_perdir_config hook.

trunk patch: http://svn.apache.org/r1368121
Submitted by: minfrin
Reviewed by: jim, sf


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1491472 13f79535-47bb-0310-9956-ffa450edef68