Add windows userdata

This is a work-in-progress branch.

This branch adds userdata support for windows systems.
That sounds like a decent goal, but I think my suggestion was misguided. I forgot that we also need to be able to retrieve runcmds (for sshinit), so doing it all during `Render` is not sufficient.

If we had a more abstract objects and a template renderer, then that'd probably work well. Maybe just leave it for now though, it's probably not that valuable in the short term.