Ifacenum ()

   Synopsis:
      Returns the interface number corresponding to the current interface
         notification

   Notes:
      The interface notification list contains all the notifications received,
         and not yet processed, as the result of an earlier SETTRIG, SETALIAS,
         SETMACRO, SETTIMER or SETHOOK statement.
      Each active interface has a unique number. This function returns the
         number of the interface that caused the current interface notification.
      If the list is empty, -1 is returned.
