scd: One new and one improved 7816 function.

* scd/apdu.c (apdu_send_direct): New arg R_SW.
* scd/command.c (cmd_apdu): Ditto.
* scd/iso7816.c (iso7816_apdu_direct): New arg R_SW.
(iso7816_general_authenticate): New.
* scd/app-nks.c (get_chv_status, get_nks_version): Pass NULL for new
arg.

iso7816_general_authenticate will be used for the PIV card support.
The new arg to iso7816_apdu_direct and apdu_send_direct allows to get
the raw status word back without the need to handle an output buffer.

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