checks if the default listener process is alive or if there is a live process with name "TTY Listener". If such a process exists,
and does not start a new process. If no such process exists, or if
starts a new listener process named "TTY Listener", and returns it.
If a REPL with I/O through
(such as a REPL started by
) is in the debugger, then by default it blocks multiprocessing. This behavior is controlled by the value of *terminal-debugger-block-multiprocessing*.