Give slings the strength damage bonus

Fired slings now get the same damage bonus from strength that melee hits
get.  This distinguishes slings from other ranged combat options, and
makes them slightly less useless in comparison.
Give slings an enchantment damage bonus

Rocks, gems and gray stones fired from a sling will now receive a damage
bonus from the enchantment of the sling, to compensate for the inability
to enchant these ammo types.

Like the sling strength damage bonus in the previous commit, this
differentiates slings from other ranged combat options, and makes them
less useless in comparison.

e.g. A starting caveman gets +3 damage from 18/01 strength and another
+2 from the enchantment of their sling, granting +5 damage for firing
their sling even before considering what is being fired, which is quite
respectable in the early game.  With gauntlets of power (+7 damage from
25 strength) and a +7 sling this becomes +14 damage for every shot
before considering ammo, skill damage and multishot bonuses, the highest
flat damage bonus out of any ranged combat option.