Bring our config_pool API up on par with the standard config API
by adding support for MUST_EXIST and CASE_SENSITIVE parameters.

This requires a similar extension to the svn_repos__retrieve_config
API that we rely on.  Since the underlying svn_config_t API already
support c/s, we simply need to pass the extra flag through to it.

The config_pool internal storage now harbors up to two alternative
config instances - one case sensitive one case in-sensitive.  In most
cases, we will read any given config with consistent c/s settings,
i.e. only one of the config entries will actually be used.

* subversion/include/private/svn_repos_private.h
  (svn_repos__retrieve_config): add c/s parameter to API
  (svn_repos__config_pool_get): add c/s and must_exist parameters

* subversion/libsvn_repos/authz.c
  (authz_retrieve_config_repo,
   svn_repos__retrieve_config): add c/s pass-through parameter 
  (svn_repos__authz_read): update caller

* subversion/libsvn_repos/config_pool.c
  (config_ref_t): use separate config struct for c/s and c/i
  (return_config_ref): add c/s parameter and return appropriate config object
  (config_by_checksum): add c/s parameter and check appropriate config object
                        for availability
  (config_add): add c/s parameter and update the correct config variant
  (auto_parse,
   add_checksum,
   find_repos_config): add c/s pass-through parameter
  (config_by_url): add c/s parameter and select the correct config variant
  (svn_repos__config_pool_get): add c/s pass-through parameter

* subversion/tests/libsvn_repos/repos-test.c
  (test_config_pool): update API caller

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1538109 13f79535-47bb-0310-9956-ffa450edef68