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.
That was #7004, merged to dev last week. It streamlined stance damage modeling into "if you have enough broken limbs, you loose the ability to stand" but I found after playing with it a bit more scaling is needed, as not being able to stand is pretty harsh.

With this PR, if you have 2+ broken body parts you can still stand but you fall over frequently.

If you have all 4 broken body parts or 2+ missing body parts (either one leg or both feet) you loose the ability to stand.

As well the slowdown now scales with stance_damage.

I should probably make sure this works correctly with wheelchairs.