Add windows userdata

This is a work-in-progress branch.

This branch adds userdata support for windows systems.
again, please don't os.Exit.

And really there's not much call for MustSucceed except in (say) the jujud/paths package, where the whole tool really depends on having this data. Generally we'll be running this on a state server and we don't want to kill that for any reason short of actual zombie apocalypse ;).