Hide shell functions from _G using upvalues on the main chunk

Store __command, __interactive, __getenv, __setenv as upvalues
populated by lua_load() from the registry, keeping them invisible
to user code while accessible to the parser's codegen.

lua.h

@@ -83,7 +83,8 @@ typedef struct lua_State lua_State;
 /* index 1 is reserved for the reference mechanism */
 #define LUA_RIDX_GLOBALS	2
 #define LUA_RIDX_MAINTHREAD	3
-#define LUA_RIDX_LAST		3
+#define LUA_RIDX_LUSH		4
+#define LUA_RIDX_LAST		LUA_RIDX_LUSH
 
 
 /* type of numbers in Lua */