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/agent.go

@@ -98,8 +98,7 @@ func (ak *AgentKey) String() string {
 	return s
 }
 
-// Key returns an agent's public key as a *rsa.PublicKey, *dsa.PublicKey, or
-// *OpenSSHCertV01.
+// Key returns an agent's public key as one of the supported key or certificate types.
 func (ak *AgentKey) Key() (interface{}, error) {
 	if key, _, ok := parsePubKey(ak.blob); ok {
 		return key, nil
@@ -204,8 +203,7 @@ func (ac *AgentClient) RequestIdentities() ([]*AgentKey, error) {
 }
 
 // SignRequest requests the signing of data by the agent using a protocol 2 key
-// as defined in [PROTOCOL.agent] section 2.6.2.  Supported key types include
-// *rsa.PublicKey, *dsa.PublicKey, *OpenSSHCertV01.
+// as defined in [PROTOCOL.agent] section 2.6.2.
 func (ac *AgentClient) SignRequest(key interface{}, data []byte) ([]byte, error) {
 	req := marshal(agentSignRequest, signRequestAgentMsg{
 		KeyBlob: serializePublickey(key),