Plate Armor Upgrade

Plate Armor has fallen behind other armors in many ways. It's only got 1
point of direct damage reduction over nerudite, and none of the other
spell-resistance benefits. If plate armor is going to be a boss at what
it does, it needs to be stronger.

Therefore, I propose this change:

* Added 10% slash, thrust, bludgeon, and pierce resistance to plate
armor.

I chose individual resistances rather than ATCK_WEAP_ALL so that the
values can stack with shields such as Gold Shields (10% slash, 10%
bludgeon, 10% thrust), Knight's Shield (10% pierce), Small Round Shield
(10% slash), and Herald Shields (10% pierce).

These new resistances will help plate armor's damage reduction scale
with our new higher damages, without dramatically affecting low damage
values (usually due to gort + lots of direct armor). So these new
resists will, for example, take 3 damage off of a 30 damage scimitar
hit, but only reduce a 10 damage hit by 1.

Hopefully, this will help plate armor become a powerful option again,
especially if combined with nerudite armor rebalancing.