When the CA loads new issuers (both their certificates and their
private keys), it performs a variety of sanity checks, such as
ensuring that the profile's signature algorithm matches the key
type.
With this change, we also check that the issuer's certificate has
the appropriate key usage bits set:
`certSign`, if it is going to be issuing end-entity certs; and
`digitalSignature`, because it will be signing OCSP responses for
previously-issued certificates.
Fixes#5068
7672d9bc99