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/common.go

@@ -53,7 +53,7 @@ var supportedHostKeyAlgos = []string{
 // This is based on RFC 4253, section 6.4, but with hmac-md5 variants removed
 // because they have reached the end of their useful life.
 var supportedMACs = []string{
-	"hmac-sha1", "hmac-sha1-96",
+	"hmac-sha2-256", "hmac-sha1", "hmac-sha1-96",
 }
 
 var supportedCompressions = []string{compressionNone}