Implement item::gun_skill to get skill used by a gun.

This differs from item::skill (previously used in ranged.cpp) as it returns the gun skill, not any skill from books or gunmods.
Item info shows data of active gunmod.

The gun section of the info shows the data taken from the active gunmod (if any), or (no active gunmod) from the gun itself.
This allows the player to see the actual values that are used to fire.