All objects are kept 'new' in incremental GC

Small changes to ensure that all objects are kept 'new' in incremental
GC (except for fixed strings, which are always old) and to make that
fact clearer.

lfunc.c

@@ -234,9 +234,10 @@ int luaF_close (lua_State *L, StkId level, int status) {
     luaF_unlinkupval(uv);
     setobj(L, slot, uv->v);  /* move value to upvalue slot */
     uv->v = slot;  /* now current value lives here */
-    if (!iswhite(uv))
+    if (!iswhite(uv)) {  /* neither white nor dead? */
       gray2black(uv);  /* closed upvalues cannot be gray */
-    luaC_barrier(L, uv, slot);
+      luaC_barrier(L, uv, slot);
+    }
   }
   return status;
 }