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>