scd:openpgp: Allow reading and writing user certs for keys 1 and 2

* scd/iso7816.c (CMD_SELECT_DATA): New.
(iso7816_select_data): New.
* scd/app-openpgp.c (do_readcert): Allow OpenPGP.1 and OPENPGP.2
(do_writecert): Ditto.
(do_setattr): Add CERT-1 and CERT-2.

This has been tested with a Zeitcontrol 3.4 card. A test with a
Yubikey 5 (firmware 5.2.6) claiming to support 3.4 failed.

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