* 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>