Draft: New mutator: Killers Spawn Faster.

Should work in most game types, and increases one's respawn delay on every death, while decreasing it when fragging someone.

Still TBD: entering cvars in cfg file, and adding mutator to menu's mutator dialog. Released for initial trying out. And probably nerf it a bit - right now it's intentionally strongly configured so it is easy to try out quickly. And find out if we need these "body" checks.