[3.12] gh-127586: multiprocessing.Pool does not properly restore blocked signals...

[3.12] gh-127586: multiprocessing.Pool does not properly restore blocked signals (try 2) (GH-128011) (#128299)

gh-127586: multiprocessing.Pool does not properly restore blocked signals (try 2) (GH-128011)

Correct pthread_sigmask in resource_tracker to restore old signals

Using SIG_UNBLOCK to remove blocked "ignored signals" may accidentally
cause side effects if the calling parent already had said signals
blocked to begin with and did not intend to unblock them when
creating a pool. Use SIG_SETMASK instead with the previous mask of
blocked signals to restore the original blocked set.
(cherry picked from commit aeb9b65aa26444529e4adc7d6e5b0d3dd9889ec2)

Co-authored-by: Stephen Hansen <stephen.paul.hansen@gmail.com>
Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>