[release-branch.go1.11] net: concatenate multiple TXT strings in single TXT record

When go resolver was changed to use dnsmessage.Parser, LookupTXT
returned two strings in one record as two different records. This change
reverts back to concatenating multiple strings in a single
TXT record.

Updates #27763
Fixes #27886

Change-Id: Ice226fcb2be4be58853de34ed35b4627acb429ea
Reviewed-on: https://go-review.googlesource.com/136955


Reviewed-by: Ian Gudger <igudger@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Ian Gudger <igudger@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit 7b3b160323b56b357832549fbab7a60d27688ec1)
Reviewed-on: https://go-review.googlesource.com/138177


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>