dirmngr: Minor code cleanup in the CRL cache.
* dirmngr/crlcache.c (INVCRL_TOO_OLD): New. (INVCRL_UNKNOWN_EXTN, INVCRL_GENERAL): New. (open_dir, crl_cache_insert): Use the new constants. (list_one_crl_entry): Make diagnostics robust for new INVCRL codes.