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

[jycr]

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.