net: fix LookupCNAME on Plan 9

net: fix LookupCNAME on Plan 9

In CL 34650, LookupCNAME was changed so it always returns
the canonical DNS host, even when there is no CNAME record.

Consequently, TestLookupCNAME was failing on Plan 9,
because www.google.com doesn't have a CNAME record.

We changed the implementation of lookupCNAME on Plan 9, so it
returns the canonical DNS host after a CNAME lookup failure.

Fixes #18391.

Change-Id: I59f361bfb2c9de3953e998e8ac58c054979210bd
Reviewed-on: https://go-review.googlesource.com/34633
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>