Some adjustments to item interesting strings

For weapons, negative bonus is now shown as "- 2" instead of as "+ -2".

For armour, the bonus is now accounted for and the numbers should match
what is actually calculated in damage_pc().

Thanks to @fosnola for noticing the armour discrepancy.

src/mathutil.hpp

@@ -35,3 +35,8 @@ inline void move_to_zero(T& val){
 	if(val > 0)
 		val--;
 }
+
+template<typename T>
+inline T sgn(T val) {
+	return val == 0 ? 0 : std::copysign(1, val);
+}