go.crypto/ssh: fix certificate parsing/marshaling.

The change to add the PublicKey interface accidentally caused certificate handling to expect an extra copy of the private key algorithm name in the binary representation. This change adapts a suitable parsing API and adds a test to ensure that cert handling isn't easily broken in the future.

R=agl, hanwen, jmpittman
CC=golang-dev
https://golang.org/cl/13272055

ssh/common.go

@@ -201,7 +201,7 @@ func serializeSignature(name string, sig []byte) []byte {
 // generating an authorized_keys or host_keys file.
 func MarshalPublicKey(key PublicKey) []byte {
 	// See also RFC 4253 6.6.
-	algoname := key.PrivateKeyAlgo()
+	algoname := key.PublicKeyAlgo()
 	blob := key.Marshal()
 
 	length := stringLength(len(algoname))