net: fix LookupCNAME on Plan 9

net: fix LookupCNAME on Plan 9

Looking up a nonexistent CNAME record on an existing
domain on Plan 9 can return either a "dns failure"
error or a "resource does not exist" error.

Fixes #21335.

Change-Id: Iead8ed4fe3167db06adb4ab7797c52c7efc3ff89
Reviewed-on: https://go-review.googlesource.com/53670
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Avelino <t@avelino.xxx>