go.crypto/ssh: some cleanup

Simplify MarshalAuthorizedKey by using the algoName func.
Make the algoName func be very specific about supported key types in openssh certs.
Generalize some of the commentary that previously mentioned specific key types.

R=agl, dave
CC=golang-dev
https://golang.org/cl/6938067

ssh/common.go

@@ -255,7 +255,21 @@ func algoName(key interface{}) string {
 			return KeyAlgoECDSA521
 		}
 	case *OpenSSHCertV01:
-		return algoName(key.(*OpenSSHCertV01).Key) + "-cert-v01@openssh.com"
+		switch key.(*OpenSSHCertV01).Key.(type) {
+		case *rsa.PublicKey:
+			return CertAlgoRSAv01
+		case *dsa.PublicKey:
+			return CertAlgoDSAv01
+		case *ecdsa.PublicKey:
+			switch key.(*OpenSSHCertV01).Key.(*ecdsa.PublicKey).Params().BitSize {
+			case 256:
+				return CertAlgoECDSA256v01
+			case 384:
+				return CertAlgoECDSA384v01
+			case 521:
+				return CertAlgoECDSA521v01
+			}
+		}
 	}
 	panic("unexpected key type")
 }