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.

lbuiltin.c

@@ -155,5 +155,9 @@ void luaopen_builtins (lua_State *L) {
   lua_setfield(L, -2, "exec");
   lua_pushcfunction(L, builtin_umask);
   lua_setfield(L, -2, "umask");
-  lua_setglobal(L, "__builtins");
+  /* Store as __lush.builtins in registry */
+  lua_rawgeti(L, LUA_REGISTRYINDEX, LUA_RIDX_LUSH);
+  lua_insert(L, -2);
+  lua_setfield(L, -2, "builtins");
+  lua_pop(L, 1);
 }