os: fix IsPermission on windows
os: fix IsPermission on windows

Fixes issue 3482.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5987070

gnanderson/go,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,tv42/old-go,gnanderson/go,apparentlymart/golang,gnanderson/go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,gnanderson/go