card: Support reading and writing PIV certificates
* scd/app-piv.c (add_tlv): New. (put_data): New. (do_writecert): New. (do_setattr): Remove usused special mode 0. * tools/gpg-card-tool.c (cmd_writecert): Allow other cards than OPENPGP. (cmd_readcert): Ditto.
- Signed-off-by: Werner Koch <wk@gnupg.org>