Lua struct obj improvements

If the core frees the obj struct referred by lua, don't free it,
just mark it as OBJ_LUAFREE - lua will free it in gc once all
the references to it are gone.

include/obj.h

@@ -66,7 +66,8 @@ struct obj {
 #define OBJ_MIGRATING 5 /* object sent off to another level */
 #define OBJ_BURIED 6    /* object buried */
 #define OBJ_ONBILL 7    /* object on shk bill */
-#define NOBJ_STATES 8
+#define OBJ_LUAFREE 8   /* object has been dealloc'd, but is ref'd by lua */
+#define NOBJ_STATES 9
     xchar timed; /* # of fuses (timers) attached to this obj */
 
     Bitfield(cursed, 1);