MkdirAll() need to use isSeparator().
Move primary defines of filepath.Separator/filepath.ListSeparator
 to os.PathSeparator/os.PathListSeparator.
Move filepath.isSeparator() to os.IsPathSeparator().
filepath package refer them from os package.
Fixes #1831.

R=rsc, alex.brainman
CC=golang-dev
https://golang.org/cl/4535100