Fixed slabfix_0gb_us to be compatible with Minetest 0.4.5.
Minetest 0.4.5's minetest_game adds new upside down slabs and stairs, which I was unaware of. The old slab fix made using upside down slabs impossible. This new version fixes that incompatability.
... of course, Minetest 0.4.6's minetest_game fixes the root of the problem, by making the slab recipe yeild six slabs as it should. Once Minetest 0.4.6 is released, there will be no more need for the slab fix, as slabs will no longer be broken.