Fixes multiple slips in a row

- you can no longer slip people who are already lying down (#1784)
- walking on cleaner foam doesn’t slip you any more
- nerfs lube. You can no longer be slipped x times in a row
- 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.
- slimes can’t slip, but aliens can.