* scdaemon.c, scdaemon.h: New option --disable-opensc.
* 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.