It turns out that there is a path that initializes netpoll and opens
file descriptors before running the os/exec init function: on some
systems, the uses of NewFile when setting os.Stdin and friends can
initialize netpoll which can open file descriptors. This in itself
is not a problem, but when we check whether the new files are open
using os.NewFile, a side-effect is to put them into non-blocking mode.
This can then break future uses of netpoll.

Updates #35469
Fixes #35566

Change-Id: I1b2e2c943695d1c2d29496b050abbce9ee710a00
Reviewed-on: https://go-review.googlesource.com/c/go/+/207078Reviewed-by: Bryan C. Mills <bcmills@google.com>