ssh: refuse to parse certificates that use a certificate as signing key

According to draft-miller-ssh-cert-01, Section 2.1.1, certificates with
certificate keys as signature keys are invalid

Change-Id: I474524ea444deb78f2fa7c2682e47c0fd057f0b8
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/678716
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

ssh/keys.go

@@ -273,7 +273,7 @@ func ParseAuthorizedKey(in []byte) (out PublicKey, comment string, options []str
 	return nil, "", nil, nil, errors.New("ssh: no key found")
 }
 
-// ParsePublicKey parses an SSH public key formatted for use in
+// ParsePublicKey parses an SSH public key or certificate formatted for use in
 // the SSH wire protocol according to RFC 4253, section 6.6.
 func ParsePublicKey(in []byte) (out PublicKey, err error) {
 	algo, in, ok := parseString(in)