common-workstation: Move SLiM stuff into a module.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
modules/slim: Don't set defaultUser anymore.

I find this annoying, especially if you want to reboot and have to
delete the default user first.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
release: Add manual for Vuizvui-specific options.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>