Clear interface between references and predefines

The reference system has a defined way to add initial values to the
table where it operates.

lua.h

@@ -80,9 +80,10 @@ typedef struct lua_State lua_State;
 
 
 /* predefined values in the registry */
-#define LUA_RIDX_MAINTHREAD	1
+/* index 1 is reserved for the reference mechanism */
 #define LUA_RIDX_GLOBALS	2
-#define LUA_RIDX_LAST		LUA_RIDX_GLOBALS
+#define LUA_RIDX_MAINTHREAD	3
+#define LUA_RIDX_LAST		3
 
 
 /* type of numbers in Lua */