Getexittwin (number)

   Synopsis:
      Returns the twin of an exit in the automapper's current room

   Notes:
      The twin exit is an exit that leads back to the current room, usually in
         the opposite direction. 'number' is the position of the exit in the
         room's ordered list of exits (the first exit is numbered 1). The return
         value is the exit model number of the twin exit.
      If there is no current room, if the numbered exit doesn't exist or if the
         exit doesn't have a twin exit, returns 0.
      To get the number of exits in the current room, see the Getroomexits ()
         function.
