Historically net.Conn.File.Fd has returned a descriptor in blocking mode.
That was broken by CL 495079, which changed the behavior for os.OpenFile
and os.NewFile without intending to affect net.Conn.File.Fd.
Use a hidden os entry point to preserve the historical behavior,
to ensure backward compatibility.

Change-Id: I8d14b9296070ddd52bb8940cb88c6a8b2dc28c27
Reviewed-on: https://go-review.googlesource.com/c/go/+/496080


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Reviewed-by: Ian Lance Taylor <iant@google.com>
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