scd:nks: Implement writecert for the Signature card v2.

* scd/iso7816.c (CMD_UPDATE_BINARY): New.
(iso7816_update_binary): New.
* scd/app-nks.c (do_deinit): Factor some code out to...
(flush_fid_cache): new.
(do_writecert): New.
(app_select_nks): Register new handler.

This has been backported only to make the following backpoorts easier.
The code is only used in 2.3; for details see the original commit
message.

Backported-from-master: c1663c690b29d2dea8bc782c42de5eca08a24cc9

• Signed-off-by: Werner Koch <wk@gnupg.org>
• GnuPG-bug-id: T6252