Returns the platform specific file descriptor for handles that are
backed by one. The datatype is abstracted as uv_os_fd_t, which maps to
int on Unices and HANDLE on Windows.
Users can use this function to set specific socket options, for example,
in a non portable way.
This function is essentially a shotgun, you better be careful with
whatever you do with it, don't blame me if you used it to get the fd of
a stream, close it yourself and expect things to Just Work.
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