This change implements cancellable lookup
on Plan 9. The query function has been modified
to return when the ctx.Done channel is closed.

Fixes #25361.

Change-Id: I544b779ceec8d69975bc7363045849c21cbfd59e
Reviewed-on: https://go-review.googlesource.com/112981
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>