Handle a basic "just units" game

[sprunk]

Here's an extremely basic game https://github.com/sprunk/peewees_vs_aks

• the units don't move. I assume it's because of the lack of LUS gadget. If that is the case, make the LUS gadget included by the default luarules handler. Or maybe movedefs failed somehow? No idea.
• the units don't shoot. I assume it's because the weapon defs don't exist, since the default posts don't parse weapons inside units. Make the default unitdefs_post do that. Perhaps even the engine?
• if something can't be done, document why.