ssh/knownhosts: compare only public key portions for revocation

Revocation matching compared full marshaled key blobs, so a @revoked
line containing a plain host key did not revoke a certificate
certifying that key: when the server presented such a certificate,
IsRevoked only matched the whole certificate blob or the CA key, and
verification succeeded even though the underlying host key was revoked.

OpenSSH's check_key_not_revoked compares keys with sshkey_equal_public,
which considers only the public portions and explicitly allows
comparisons between certificates and plain keys. Match that behavior by
normalizing both @revoked entries and lookups to the underlying public
key: for a certificate, the certified key rather than the certificate
blob. This also makes a @revoked line containing a certificate revoke
the plain key it certifies

Change-Id: I3029d59e68fb01f2340763e2eae25cac23c6193d
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/816841
Reviewed-by: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>