[v0.27] Strength Modification for Armour Bug

[kabill]

The Strength boost variable for armour seems to work using the vanilla system of setting the value rather than adding to the soldier's existing value. All the others work fine, it's just strength that does this.

[Solver]

This is for compatibility with vanilla, because the Predator sets strength to 100, doesn't add 100 to strength. So this is a bit inconsistent, but a known thing to maintain the Predator's behaviour.