[issue16500] Allow registering at-fork handlers

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

`atexit.register()` has different signature and allows to pass arbitrary positional and keyword arguments to the registered function. This is incompatible with the "when" argument. If we want to support registering arguments with the function, we need either three registering functions or make the "when" parameter the first and positional-only.

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nosy: +serhiy.storchaka

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue16500>
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