adjust XP costs for traits

Eureka!  I've been irritated for years that all gnomes are
body-builders.  There's no good reason for it, but there is a bad
reason; the system.  The system tells gnomes that to raise to their
maximum Strength of +1, with so much muscle that they'll be on equal
footing with humans, despite being half their height, they only need to
spend 20 XP.  5 XP gets them from Strength -2 to Strength -1, and so on.

And almost as bad; gnomes are no more dexterous than anyone else after a
few months on the road.  They only start with +1 Dexterity, but soon
everyone buys a couple of dots in Dexterity, making everyone the same.
The racial limits really did feel like *limits*, since you just run into
them.

One trick fixes everything: racial Attribute adjustments apply to XP
costs, so gnomes must spend 10 XP to raise their Strength to -1, while
humans need to spend 10 XP to raise their Wits to 0.