* scd/app-nks.c (struct fid_cache_s): New.
(struct app_local_s): Add field 'fid_cache'.
(do_deinit): Release the cache.
(keygripstr_from_pk_file): Implement the cache.
(find_fid_by_keyref): New
(do_sign, do_decipher): Use new function.
(do_with_keygrip): New.