Addtwinexit (number)

   Synopsis:
      Converts a single exit into a two-way exit.

   Notes:
      A two-way exit might link rooms A and B together, with a "north" exit from
         room A leading to room B, and a "south" exit from room B leading to
         room A.

      This function converts a single exit (such as a one-way exit) into a
         two-way exit, creating a brand new exit in the opposite direction.
         (Axmud stores two-way exits as two separate, but linked, exits.)
      If an exit in the opposite direction already exists, then that exit is
         used instead.

      The original exit must already be connected to a room.
      If the operation is succesful, the new exit's exit model number is
         returned. If an exit in the opposite direction already existed, than
         that exit's number is returned. If the operation fails, 0 is returned.

      Note that separate Addexit() and Addtwinexit() functions give your
         scripts the opportunity to check for the failure of the former, before
         proceeding with the latter.
