Fixes slipping when lying down

- you can no longer slip people who are already lying down (#1784)
- nerfs lube. You can no longer be slipped x times in a row. Spacing people by lubing the whole hallway to escape doesn't work.
- when you are slipped by lube, you drop your items a few tiles before the turf where you fall down. I can look into changing this if requested, but it will take a more hacky approach.
- slimes and aliens don't slip, hulks only slip on lube
- walking on cleaner foam doesn’t slip you any more