Implement os.fork on posix

[slozier]

I have no idea if implementing os.fork is even feasible.

I'm filing this because some tests (test_uuid, test_ssl) are failing on posix when they try to call os.fork. Note that CPython changed the guards to check for fork instead of posix so we can backport that fix (done in ce3fceb).

Related issues: IronLanguages/ironpython2#22