scd:nks: Allow retrieving certificates from a Signature Card v.20

* scd/app-nks.c: Major rework to support non-RSA cards.

This is a fist step so support this ECC card. The code has been
reworked while taking care that old cards should keep on working.

Backported-from-master: f05a32e5c9db7d0840c74fccc350a9e0ff5fb819

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