This routine is used with the built-in procedure CALL_USER and
    its associated call-user routine to control the DECTPU access
    to the terminal. When a call-user routine invokes TPU$CLOSE_
    TERMINAL, DECTPU closes its channel to the terminal and the
    channel of the DECTPU associated mailbox.

    When the call-user routine returns control to it, DECTPU
    automatically reopens a channel to the terminal and redisplays
    the visible windows.

    A call-user routine can use TPU$CLOSE_TERMINAL at any point in
    the program and as many times as necessary. If the terminal is
    already closed to DECTPU when TPU$CLOSE_TERMINAL is used, the
    call is ignored.