signature of _posixsubprocess._fork_exec changed

[mattip]

The interface in _posixsubprocess._fork_exec changed in PRs python/cpython#91490 (add a way to disable VFORK) and python/cpython#23930 (add process_group support). We need to adapt these in the _posixsubprocess module.

[mattip]

Note that PyPy never calls vfork, so it is always "disabled".

[mattip]

Done in bea43c7