Add support for REGISTRY: urls as config / authz path to match the
"plain" svn_config* / svn_repos_authz* functionality.  For now, we
simply and inefficiently fall back to the "plain" functionality.

* subversion/include/private/svn_repos_private.h
  (svn_repos__config_pool_get,
   svn_repos__authz_pool_get): 'REGISTRY:' urls are now supported

* subversion/libsvn_repos/config_pool.c
  (svn_repos__config_pool_get): Make sure we set *KEY to a defined value
                                if we don't know a key

* subversion/libsvn_repos/authz_pool.c
  (svn_repos__authz_pool_get): Fall back to non-cached reader in case
                               we run into any special case

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1541948 13f79535-47bb-0310-9956-ffa450edef68