and/or change the signal mask of the calling thread.

Fix also tests of test_io using threads and an alarm: use pthread_sigmask() to
ensure that the SIGALRM signal is received by the main thread.

Original patch written by Jean-Paul Calderone.

Possibly redundant once 3.2.1 is out.

merge from 3.2.

constants to be used for STARTUPINFO were exposed due to the change.

before continuing.

This is useful for starting up the test suite to attach a debugger such
as Visual Studio or others.

by Charles-François Natali.

and make it available as `test.support.IPV6_ENABLED`.  Patch by
Charles-François Natali.

Initial patches by Tobias Brink. Thanks!