* card.c (card_open): Implement it.
* apdu.c (open_osc_reader, osc_send_apdu): New.
(apdu_open_reader) [HAVE_OPENSC]: Use the opensc driver if not
disabled.
(error_string) [HAVE_OPENSC]: Use sc_strerror.
(send_apdu) [HAVE_OPENSC]: Call osc_apdu_send.
* card-p15.c (p15_enum_keypairs, p15_prepare_key): Adjusted for
libgpg-error.