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