Honour mandatory digest on private key in has_usable_cert()

If the private key says it can only support one specific digest, then
don't ask it to perform a different one.

Fixes: #7348

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7408)