Half_physical_damage 07

It is not physical damage if:
1. it already qualifies for some other special type of damage
   for which a special resistance already exists in the game
   including: cold, fire, shock, and acid. Note that fire is
   extended to include all forms of burning, even boiling water
   since that is already dealt with by fire resistance, and in
   most or all cases is caused by fire.
2. it doesn't leave a mark. Marks include destruction of, or
   damage to, an internal organ (including the brain),
   lacerations, bruises, crushed body parts, bleeding.

Current exceptions to the rule (already existing):
- holy water burning chaotic ("it burns like acid") is physical damage.
- unholy water burning lawful is physical damage.

src/explode.c

@@ -71,8 +71,8 @@ int expltype;
 		case 1: str =   olet == BURNING_OIL ?	"burning oil" :
 				olet == SCROLL_CLASS ?	"tower of flame" :
 							"fireball";
+			/* fire damage, not physical damage */
 			adtyp = AD_FIRE;
-			physical_dmg = TRUE;
 			break;
 		case 2: str = "ball of cold";
 			adtyp = AD_COLD;