ssh: Add the hmac-sha2-256-etm@openssh.com algorithm Fixes golang/go#17676 Change-Id: I96c51431b174898a6bc0f6bec7f4561d5d64819f Reviewed-on: https://go-review.googlesource.com/35513 Reviewed-by: Han-Wen Nienhuys <hanwen@google.com> Run-TryBot: Han-Wen Nienhuys <hanwen@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org>