bpo-35823: subprocess: Fix handling of pthread_sigmask() errors (GH-22944)

bpo-35823: subprocess: Fix handling of pthread_sigmask() errors (GH-22944)

Using POSIX_CALL() is incorrect since pthread_sigmask() returns
the error number instead of setting errno.

Also handle failure of the first call to pthread_sigmask()
in the parent process, and explain why we don't handle failure
of the second call in a comment.