os.NewFile doesn't put the fd into non-blocking mode.
In most cases, an *os.File returned by os.NewFile is in blocking mode.

Updates #7970
Updates #21856
Updates #23111

Change-Id: Iab08432e41f7ac1b5e25aaa8855d478adb7f98ed
Reviewed-on: https://go-review.googlesource.com/83995Reviewed-by: Ian Lance Taylor <iant@golang.org>