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/server.go

@@ -406,7 +406,7 @@ userAuthLoop:
 					break
 				}
 				signedData := buildDataSignedForAuth(H, userAuthReq, algoBytes, pubKey)
-				key, _, ok := parsePubKey(pubKey)
+				key, _, ok := ParsePublicKey(pubKey)
 				if !ok {
 					return ParseError{msgUserAuthRequest}
 				}