[issue1378] fromfd() and dup() for _socket on WIndows

[Christian Heimes]

Christian Heimes added the comment:

I've reviewed your patch and merged it was some pending changes of my
own. The socket tests are passing on Windows. Great work :)

UPDATE:
* Added PyLong_FromSocket_t and PyLong_AsSocket_t to longobject.h to get
rid of some 64bit warnings
* Use name dup_socket() instead of dup(). For Windows developers it's
too confusing to name it dup().
* Readded fromfd, but this time in C again.

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