Fix ForkedFunc to handle hard exit(0)

[vermaport]

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1. waitfinish() assumes that if the forked child process exited with exitstatus == 0, that _child() ran to completion and wrote out the retval. This is not true if the function run in the child process hard exited with status 0, e.g. os._exit(0) (or even sys.exit(0)).

2. Add new unit test test_exit0() for this case.

3. Fixes #261