Fixed a few bugs relating to stairs while sneaking

https://github.com/minetest/minetest/issues/329
I was looking through the issue, saw that bug, decided to check it out. Turns out there was also an issue where sneaking while on a slab caused you to hover over the slab. This fixes both.

What I did was, in LocalPlayer::move, I changed four references to BS to BS/2. From what I can tell, it doesn't break anything and fixes the bug, but I'm not very familiar with the codebase.