gpg: New option --use-only-openpgp-card

* g10/gpg.c (opts): Add option.
(main): Set flag.
* g10/options.h: Add flags.use_only_openpgp_card.
* g10/call-agent.c (start_agent): Implement option.

With the previous patch we switch to autoselect an application
instead of requesting an openpgp card. This option allows to revert
this in case of use use cases which expected the former behaviour.

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