The path package now contains only functions which
deal with slashed paths, sensible for any OS when dealing
with network paths or URLs.  OS-specific functionality
has been moved into the new path/filepath package.

This also includes fixes for godoc, goinstall and other
packages which were mixing slashed and OS-specific paths.

R=rsc, gri, mattn, brainman
CC=golang-dev
https://golang.org/cl/4252044