oextra follow-up - merging (trunk only)

It seemed inappropriate to allocate and tack on an oextra
structure just to mark it as unmergable, and the oextra
struct itself wouldn't be released until a save/restore took
place.

This uses one of the freed up oattached bits to prevent the merge.

src/eat.c

@@ -297,9 +297,9 @@ register struct obj *otmp;
 		dropy(otmp);
 		sellobj_state(SELL_NORMAL);
 	    } else {
-		newolong(otmp);		/* hack to prevent merge */
+		otmp->nomerge = 1;	/* used to prevent merge */
 		otmp = addinv(otmp);
-		free_olong(otmp);
+		otmp->nomerge = 0;
 	    }
 	}
 	return(otmp);

src/invent.c

@@ -2416,6 +2416,9 @@ mergable(otmp, obj)	/* returns TRUE if obj  & otmp can be merged */
 	    obj->bypass != otmp->bypass)
 	    return(FALSE);
 
+	if (obj->nomerge)	/* explicitly marked to prevent merge */
+	    return FALSE;
+
 	if ((obj->oclass==WEAPON_CLASS || obj->oclass==ARMOR_CLASS) &&
 	    (obj->oerodeproof!=otmp->oerodeproof || obj->rknown!=otmp->rknown))
 	    return FALSE;