scd:openpgp: New attribute "MANUFACTURER".

* scd/app-openpgp.c (get_manufacturer): New..
(do_getattr): Add new attribute "MANUFACTURER".
(do_learn_status): Always print it.

This will make it easy to maintain the list of OpenPGP vendors at just
one place.

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