gpg: Let --show-keys print revocation certificates.

* g10/import.c (list_standalone_revocation): New.
(import_revoke_cert): Call new function.

(cherry picked from commit 386b9c4f25b28fd769d7563f2d86ac3a19cc3011)

• GnuPG-bug-id: T4018
• Signed-off-by: Werner Koch <wk@gnupg.org>