ssh: add hmac-sha2-256.

Fixes golang/go#10274

Change-Id: Id8386828ee92ccc6cba5197831cdb8b2ce0cd648
Reviewed-on: https://go-review.googlesource.com/8353
Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>

ssh/mac.go

@@ -9,6 +9,7 @@ package ssh
 import (
 	"crypto/hmac"
 	"crypto/sha1"
+	"crypto/sha256"
 	"hash"
 )
 
@@ -44,6 +45,9 @@ func (t truncatingMAC) Size() int {
 func (t truncatingMAC) BlockSize() int { return t.hmac.BlockSize() }
 
 var macModes = map[string]*macMode{
+	"hmac-sha2-256": {32, func(key []byte) hash.Hash {
+		return hmac.New(sha256.New, key)
+	}},
 	"hmac-sha1": {20, func(key []byte) hash.Hash {
 		return hmac.New(sha1.New, key)
 	}},