os: if descriptor is non-blocking, retain that in Fd method

For #58408
Fixes #60211

Change-Id: I30f5678b46e15121865b19d1c0f82698493fad4e
Reviewed-on: https://go-review.googlesource.com/c/go/+/495079


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Reviewed-by: Bryan Mills <bcmills@google.com>
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