ssh/terminal: add darwin support.

terminal contains a number of utility functions that are currently only
implemented for Linux. Darwin uses different named constants for
getting and setting the terminal state so this change splits them off
as constants and defines them for each arch.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/7286043

ssh/terminal/util_linux.go

@@ -0,0 +1,12 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build linux
+
+package terminal
+
+import "syscall"
+
+const ioctlReadTermios = syscall.TCGETS
+const ioctlWriteTermios = syscall.TCSETS