internal/poll, os/exec, runtime: replace PollDescriptor by IsPollDescriptor

internal/poll, os/exec, runtime: replace PollDescriptor by IsPollDescriptor

This commit changes poll.PollDescriptor by poll.IsPollDescriptor. This
is needed for OS like AIX which have more than one FD using inside their
netpoll implementation.

Change-Id: I49e12a8d74045c501e19fdd8527cf166a3c64850
Reviewed-on: https://go-review.googlesource.com/c/146938
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>