* scd/command.c (cmd_learn): Add flag --reread.
* scd/app-common.h (struct app_ctx_s): New field need_reset.
* scd/app.c (write_learn_status_core): Set need_reset if we notice an
error after returning from a reread.  Change all callers of card
functions to return GPG_ERR_CARD_RESET so that that app is not anymore
used.