g10,scd: Improve UIF support.
* g10/call-agent.c (learn_status_cb): Parse "bt" flag. * g10/call-agent.h: New member field "bt". * g10/card-util.c (uif): Limit its access only when it is supported. * scd/app-openpgp.c (do_setattr): Allow access to UIF objects only when there is a button.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>