Fix an issue of concurrent.futures ProcessPoolExecutor shutdown hanging.

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

This reverts the core of #100618 while leaving relevant documentation
improvements and minor refactorings in place.

This starts the process. Users who don't specify their own start method
and use the default on platforms where it is 'fork' will see a
DeprecationWarning upon multiprocessing.Pool() construction or upon
multiprocessing.Process.start() or concurrent.futures.ProcessPool use.

See the related issue and documentation within this change for details.

Co-authored-by: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com>

Do not spawn ProcessPool workers on demand when they spawn via fork.

This avoids potential deadlocks in the child processes due to forking from
a multithreaded process.

Prevent `max_tasks_per_child` use with a "fork" mp_context to avoid deadlocks.

Also defaults to "spawn" when no mp_context is supplied for safe convenience.

Do not store `ProcessPoolExecutor` work item exception traceback that prevents
exception frame locals from being garbage collected.