* modules/aaa/mod_authz_owner.h: Add header file with optional hook

* modules/aaa/mod_authz_owner.h: Add header file with optional hook
  declaration for "authz_owner_get_file_group".

* modules/aaa/mod_authz_dbm.c, modules/aaa/mod_authz_groupfile.c: Use
  the header to pick up the above declaration; retrieve the optional
  function in a hook; use a static variable to store the function
  pointer.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1420184 13f79535-47bb-0310-9956-ffa450edef68