scd: Add option --multi to the LEARN command.

* scd/app-common.h (APP_LEARN_FLAG_MULTI): New.
* scd/command.c (cmd_learn): Add option --multi.
* scd/app.c (app_write_learn_status): Factor some code out to ...
(write_learn_status_core): new.
(app_write_learn_status): Implement flag --multi.

This new option is intended to return information about all active
applications of the current card. Thus if a "SERIALNO openpgp" and a
"SERIALNO piv" has been done in a session the command "LEARN --force
--multi" returns information about both applications.

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