Message 372426 - Python tracker

With issue35537 merged, do we have any intention to get on with this? From what I can tell, it provides roughly the same performance benefit - but works also with close_fds=True, which is the default of Popen, so IMO it's a worthy addition.

I tested a rebase on current master, test_subprocess passes on my Linux box and there are no more -Wclobbered warnings after Alexey's latest change of the do_fork_exec() helper.