* scd/command.c (pincache_put): New.  Uses a dummy key for now.
(pincache_get): New.
* scd/app.c (select_application): Flush the PIN cache.
(scd_update_reader_status_file): Ditto.
(maybe_switch_app): Call the new prep_reselect function.
(app_write_learn_status): Ditto.
* scd/app-openpgp.c (cache_pin): New helper to cache a PIN.
(verify_chv2): Call it.
(verify_chv3): Call it.
(clear_chv_status): Call it.
(do_change_pin): Call it.
* scd/app-common.h (struct app_ctx_s): Add function 'prep_select'.
* scd/app-openpgp.c (do_prep_reselect): New stub function.
(app_select_openpgp): Set new stub function.
* scd/app-piv.c (do_prep_reselect): New stub function.
(app_select_piv): Set new stub function.
* scd/app-common.h (struct app_ctx_s): Add parameter ctrl to setattr,
sign, auth, decipher, and check_pin.  Change all implementations and
callers to pass such a parameter.