If we get an EAGAIN error on an unpollable file, don't try to wait for
it to be ready; just return EAGAIN.

It's possible that we should instead ensure that when Stdin is a pipe
in non-blocking mode, we wait for data to appear. For now take the
conservative approach of doing what we did in previous releases.

Based on https://golang.org/cl/47555 by Totoro W.

Fixes #20915

Change-Id: Icc9e97a5a877b0a3583ec056c35412d1afab62d1
Reviewed-on: https://go-review.googlesource.com/48490
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>