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>