Implemented Parry Bonuses

Parry bonuses for weapons were never implemented. They were left as a
TODO stub, only returning exactly the user's level of Parry skill. I
created a piParryBonus for weapons and set a few values on the game's
least-used weapons.

- Short sword now acts as if player has 50% parry at all times, adding
100 defense. Great for newbies, horrible for built characters, who will
actually parry worse. Maybe this will help the 'short sword fighting
imping agony' many players suffer through. (Note - Parry bonus will not
apply if held or blinded, as usual)

- Scimitar can add up to 200 defense based on the wielder's agility,
perhaps giving it an advantage over fencing weapons. 200 is equal to the
defense penalty of wearing a plate armor, and is reached at 50 natural
agility.

- Riija swords won't parry for non-owners.

- Gold swords add 150 defense if in the arena. Seems fair for a weapon
that doesn't have procs and can't be used in real combat.

Aside from these weapons, piParryBonus should allow admins to modify
event items' parry ability, and the value will of course exist for
future balancing.