Use the new object pool API to implement an authz pool.

This requires a few minor extensions to existing authz and config APIs
to make the respective functionality available where needed.

* subversion/include/private/svn_repos_private.h
  (svn_repos__authz_pool_t,
   svn_repos__authz_pool_create,
   svn_repos__authz_pool_get): declare new private API

* subversion/libsvn_repos/repos.h
  (svn_repos__authz_validate): publish private API

* subversion/libsvn_repos/authz.c
  (svn_authz_t): update commentary mentioning a new dependency
  (authz_validate): rename to ...
  (svn_repos__authz_validate): ... this
  (svn_repos__authz_read,
   svn_repos_authz_parse): update existing callers

* subversion/include/private/svn_subr_private.h
  (svn_config__shallow_replace_section): declare new utility API

* subversion/libsvn_subr/config_file.c
  (svn_config__shallow_replace_section): implement

* subversion/libsvn_repos/authz_pool.c
  (): new file
  (svn_authz_t
   authz_object_t
   svn_repos__authz_pool_t): define data types
  (construct_key,
   getter): utility functions
  (svn_repos__authz_pool_create,
   svn_repos__authz_pool_get): implement the new authz pool API


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1540594 13f79535-47bb-0310-9956-ffa450edef68