corpses on ice use of obj recharged field

A while back, I noticed that there was a custom use of the obj->recharged
flag in mkobj.c for tracking corpses on ice.  It's much more obvious for
those of us that don't have the entire source base memorized to follow the
usual convention of adding a #define to obj.h.  That's what this change does.

include/obj.h

@@ -78,6 +78,7 @@ struct obj {
 #define opoisoned otrapped	/* object (weapon) is coated with poison */
 
 	Bitfield(recharged,3);	/* number of times it's been recharged */
+#define on_ice recharged	/* corpse on ice */
 	Bitfield(lamplit,1);	/* a light-source -- can be lit */
 #ifdef INVISIBLE_OBJECTS
 	Bitfield(oinvis,1);	/* invisible */