Handle a basic "just units" game

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.

Reproduce the basic game in https://github.com/sprunk/peewees_vs_aks and inspect the default luarules handler and default unitdefs_post. Check whether the missing LUS gadget prevents movement and whether weapon definitions inside units are ignored. Done means determining how to make the units move and shoot, or documenting why either behavior cannot be supported.

Written by the indexing model from the issue text.