smimesign --list-keys must display only valid certificates for digitalSignature

When invoke `smimesign --list-keys`, it display all certificates including :

- expired certificates
- revoked certificates
- certificates without digitalSignature key usage (cf. https://www.rfc-editor.org/rfc/rfc5280#section-4.2.1.3)

Expired/revoked certificates, and certificates without digitalSignature key usage must not be displayed.

Start at the implementation of the `smimesign --list-keys` command and trace how certificates are selected for display. Use RFC 5280 section 4.2.1.3 to verify the digitalSignature requirement, and check how expiration and revocation are represented. Done means the command displays only unexpired, unrevoked certificates with digitalSignature key usage.

Scritto dal modello di indicizzazione a partire dal testo della issue.