Reduced chance of combat skill rust for post-threshold predators

The reduction in combat skill rust rate for various tiers is equal to
the existing learning bonus they get, calculated as follows:

| Mutation | Learn bonus | Learn rate | Bonus/total |
| -------- | -----------:| ----------:| -----------:|
| PRED2    |         33% |       133% |         25% |
| PRED3    |        100% |       200% |         50% |
| PRED4    |        200% |       300% |         67% |

So this gives PRED2 a 25% chance to avoid combat skill rust, PRED3 a
50% chance to avoid it, and PRED4 a 67% chance to avoid it.

Going by the comments in mutation.cpp about the PRED3/PRED4 int
penalties, it's not so much that they're becoming stupid as it is
their brains are focusing on hunting and paying less attention to
those other trivial things like using machines or building things.

Given that, it makes sense that they'd be more likely to remember that
which they're specifically optimized for.

Note that anyone with PRED3 or PRED4 is taking a 1-3 point int penalty
(and therefore rust penalty to everything in int-based rust modes) and
that half of the PRED4 categories also include forgetful, so this
isn't as significant a boost as it might appear at first glance.

When they pass this check, it very rarely also prints a message that
gives (one point, just for fun) stim. This averages once every 8/4/3
hours for PRED2/PRED3/PRED4.
Merge branch 'posthuman-predators-dont-forget-the-hunt' of https://github.com/rluddy/Cataclysm-DDA into merge
Clean up some paren-warnings

Looks OK otherwise.