Fix butchering difficulty with Survival skill < 3.

Now that arguments to `rng()` can be in any order, this code can be cleaned up slightly. (It was already able to accept negative arguments.)

I suspect the sSkillLevel adjustment was copy-pasted from the dexterity adjustment and someone forgot to change the 8 to a 3. Prior to this change, here's how skill_shift was affected, on average, for different levels of the Survival skill:

* `level 0: -4.0 (-rng(0, 8))`
* `level 1: -3.5 (-rng(0, 7))`
* `level 2: -3.0 (-rng(0, 6))`
* `level 3: +1.5 (+rng(0, 3))`
* `level 4: +2.0 (+rng(0, 4))`

That inflection point between levels 2 and 3 shouldn't be there. (Or, if it's actually intended, I think it should be documented in-game, since it's significant.) After this change, the average adjustment is -1.5 at level 0 and increases by 0.5 with each level (making it 0 at level 3).