Adjusts damaged stance system

Makes the system for handling damaged stance behave a bit closer to what's on master, and more forgiving of broken legs and feet.

Having two broken legs/feet will now only make you fall over occasionally, like on master.
Having two missing feet or one missing leg (and hence missing foot) will stop you from standing. 

If both legs *and* both feet are broken you will still be unable to stand, but that's fine since you would be pretty messed up anyways.

The movement penalty for damaged legs/feet also scales better.
Actually, maybe this should be named stanceloss for consistency.