runtime: change fcntl to return two values

runtime: change fcntl to return two values

Separate the result and the errno value, rather than assuming
that the result can never be negative.

Change-Id: Ib01a70a3d46285aa77e95371cdde74e1504e7c12
Reviewed-on: https://go-review.googlesource.com/c/go/+/496416
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>