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