syscall: fix windows value of IPV6_V6ONLY
syscall: fix windows value of IPV6_V6ONLY

R=golang-dev, adg
CC=golang-dev
http://codereview.appspot.com/2213048
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go