Lower the range of dirt guns spawn with

Very very few guns should be fouled heavily, or fouled at all. However,
because the amount of dirt guns spawn with is simply a number between
the min an max defined here, having a range this high leads to most guns
being fouled or fairly heavily fouled.

Adjust this range back to what is was previously, which leads to most
guns being very lightly fouled or not noticeably fouled.
Itemgroups can still override this, and make guns spawn with more
fouling.

src/item_group.cpp

@@ -195,7 +195,9 @@ void Single_item_creator::inherit_ammo_mag_chances( const int ammo, const int ma
 Item_modifier::Item_modifier()
     : damage( 0, 0 )
     , count( 1, 1 )
-    , dirt( 0, 9999 ) // min and max should follow max_dirt in ranged.cpp
+      // Dirt in guns is capped unless overwritten in the itemgroup
+      // most guns should not be very dirty or dirty at all
+    , dirt( 0, 500 )
     , charges( -1, -1 )
     , with_ammo( 0 )
     , with_magazine( 0 )