Weapon fixes

* Fixed killicons for all DarkRP weapons.
* Fixed M4 iron sights in some situations (yes, again)
* Moved to GMod's wep:GetHoldType - removes DarkRP's wep.CurHoldType.
* Refactored stunstick - no more duplicate code and bugs that have been long fixed in the arrest and unarrest sticks.
* Fixed unarrest stick range being too high (was previously fixed for arrest sticks - #1109).
I think I'll change this to using the distance returned by `getEyeSightHitEntity`, after all that should give the smallest distance between the entity and the player.