various fixes/enhancements

grenade blasts now have kill / maim / stun / distract radii. distract means
that nearby zombies will go check out the explosion before resuming the hunt
for braaaaaaains

a few bugfixes and performance tweaks... avoid sqrt whenever possible.
since we never need the true distance, the square is fine.

preparation for moar physics (e.g.
thrown grenades bouncing off zombies)