H206 - passive fire damage affecting weapons (trunk only)

<email deleted> reported back on 8/31/06 that elven weapons were not
affected when he poked a fire elemental with them.  This is true, but
moreover, there was no code to have passive fire to affect attackers.
Now erode_obj() supports all the same damage types as rust_dmg(), and added
the connecting code to allow passive fire attacks do something.
There probably should be macros for the damage types used by rust_dmg
and erode_obj, and possibly these functions should be combined, but they
are slightly different and dealing with that requires more thought.

src/potion.c

@@ -1934,7 +1934,7 @@ dodip()
 	}
 
 	if (potion->otyp == POT_ACID) {
-	    if (erode_obj(obj, TRUE, FALSE, TRUE))
+	    if (erode_obj(obj, 3, FALSE, TRUE))
 		goto poof;
 	}