Publish authz_retrieve_config as svn_repos__retrieve_config
(we will need it soon).

* develop/trunk/subversion/include/private/svn_repos_private.h
  (svn_repos__retrieve_config): declare as private API function

* develop/trunk/subversion/libsvn_repos/authz.c
  (authz_retrieve_config): rename to ...
  (svn_repos__retrieve_config): ... this
  (svn_repos__authz_read): update caller

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1532201 13f79535-47bb-0310-9956-ffa450edef68