unix: Add [RWX]_OK constants for Access

POSIX defines [FRWX]_OK flags as the second arg to access() (Access()
here). Only F_OK was defined heretofore. This adds the missing
[RWX]_OK constants.

Fixes golang/go#6262

Change-Id: Ie96273db19aa21af92c46affea9509afe0efe63f
Reviewed-on: https://go-review.googlesource.com/7852Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/constants.go

+// Copyright 2015 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 darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package unix
+
+const (
+	R_OK = 0x4
+	W_OK = 0x2
+	X_OK = 0x1
+)