[issue1798] Add ctypes calling convention that allows safe access of errno

[Amaury Forgeot d'Arc]

Amaury Forgeot d'Arc added the comment:

I like this way to tell that a function modifies errno or GetLastError.

But this thread-local attribute on the function seems bizarre to me.
I would prefer another way to get the errno. I can see two alternatives:
- the function returns a tuple (normalresult, errno) on each call.
- when errno is not zero, EnvironmentError (or WindowsError) is raised.

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nosy: +amaury.forgeotdarc

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<http://bugs.python.org/issue1798>
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