Authentication was added to the :mod:`multiprocessing` forkserver start

method control socket so that only processes with the authentication key

generated by the process that spawned the forkserver can control it. This

is an enhancement over the other :gh:`97514` fixes so that access is no

longer limited only by filesystem permissions.

The file descriptor exchange of control pipes with the worker forked worker

process now requires an explicit acknowledgement byte to be sent over the

socket after the exchange on all forkserver supporting platforms. That makes

testing the above much easier.