ssh: add support for FIPS mode

Unsupported algoritms are silently ignored and not negotiated, or
rejected

Fixes golang/go#75061

Change-Id: I08d50d10a97c08e78aedead89ca61beceff88918
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/698795
Reviewed-by: Mio Mio <miomio0086@gmail.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

ssh/certs_test.go

@@ -338,7 +338,9 @@ func TestCertTypes(t *testing.T) {
 				CertType: UserCert,
 				Key:      priv.PublicKey(),
 			}
-			cert.SignCert(rand.Reader, priv)
+			if err := cert.SignCert(rand.Reader, priv); err != nil {
+				t.Fatalf("error signing certificate: %v", err)
+			}
 
 			certSigner, err := NewCertSigner(cert, priv)
 			if err != nil {