Moves the copying of curtin_userdata and preseeds out of maniacs-setup and into packaging via postinst script.
This ensures that upgrading the package will also upgrade these files preventing a nasty situation where users may upgrade MCS repeatedly, but never re-run maniacs-setup and thus end up with stale, broken curtin_userdata files.
It does prompt the user after installation to run maniacs-setup to update.
If users have configured a local mirror, of course, this will break mirror usage until they re-run maniacs-setup --update-preseeds, but that breakage will be a lot more obvious than the breakage that occurs when the preseeds are downrev compared to the rest of the mcs files.