scd: Return a stable list with "getinfo card_list".

* scd/app.c (compare_card_list_items): New.
(app_send_card_list): Sort the card objects by slot.

This is required so that in gpg-card a "list N" command always returns
the expected card. Sorting by slot should be sufficient.

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