Add facilities in the authz lookup library to perform a global lookup, that is to check whether a user has a given access level granted by any authz rule. This is the first stage of fixing issue #2388.
* subversion/include/svn_repos.h (svn_repos_authz_check_access): Document the possibility of passing a NULL path to perform a global access lookup.
* subversion/libsvn_repos/authz.c (svn_repos_authz_check_access): Test for NULL paths and perform a global access lookup if necessary. (authz_get_global_access, authz_global_parse_section): New functions.
* subversion/tests/libsvn_repos/repos-test.c (authz): Add tests to validate the behaviour of global lookups.