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.
- Signed-off-by: Werner Koch <email@example.com>
- GnuPG-bug-id: T6252