sm: New command --show-certs

* sm/keylist.c (do_show_certs): New.
(gpgsm_show_certs): New.
* sm/gpgsm.c (aShowCerts): New.
(opts): Add --show-certs.
(main): Call gpgsm_show_certs.

I have been using libksba test programs for countless times to look at
certificates and I always wanted to add such a feature to gpgsm. This
is simply much more convenient.

• Signed-off-by: Werner Koch <wk@gnupg.org>