native get_user_menu(index, &id, &keys);
Variable to store menu id to
Variable to store menu keys to
Returns if the client is watching a menu.
If there is no menu, the id is 0. If the id is negative, then the client
views a VGUI menu. Otherwise, the id is an id acquired from the
1 if client views a menu, 0 otherwise
If the client index is not within the range of 1 to
MaxClients, an error will be thrown.