Patch written by Charles-Francois Natali.

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.