sm: Support import and verification of EdDSA certificates.

* sm/certdump.c (gpgsm_get_serial): New.
* sm/certcheck.c (gpgsm_check_cert_sig): Support EdDSA signatures.

Note that this does not work with the self-signed RFC-8410 sample
certificate; see the code for comments.

The Ed488 case has not been tested due to a lack of support in
Libgcrypt.

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