scd: Simplify saving application context.
* scd/app.c (lock_table): Remove LAST_APP field. (lock_reader, app_dump_state, application_notify_card_reset) (release_application): Follow the change. (check_conflict): New. (check_application_conflict): Lock the slot and call check_conflict. (select_application): Call check_conflict and not use LAST_APP.
We don't need LAST_APP field but just keep the application context by
APP field. Since we have a reference counter, it is possible if we
can deallocate or not.