Message 372429 - Python tracker

I'd really like to get this merged eventually because vfork()-based solution is fundamentally more generic than posix_spawn(). Apart from having no issue with close_fds=True, it will also continue to allow subprocess to add any process context tweaks without waiting for availability of corresponding features in posix_spawn().

If there is no objection from Gregory or other devs, I can pick up from where I left in two weeks.