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>

src/net/lookup_plan9.go

@@ -198,7 +198,7 @@ func (*Resolver) lookupPort(ctx context.Context, network, service string) (port
 func (*Resolver) lookupCNAME(ctx context.Context, name string) (cname string, err error) {
 	lines, err := queryDNS(ctx, name, "cname")
 	if err != nil {
-		if stringsHasSuffix(err.Error(), "dns failure") {
+		if stringsHasSuffix(err.Error(), "dns failure") || stringsHasSuffix(err.Error(), "resource does not exist; negrcode 0") {
 			cname = name + "."
 			err = nil
 		}