The new appending behavior is required for an efficient DNS client.

(*Builder).Start was intended to append, but didn't. This was a mistake
(all tests and examples assumed that it did).

In addition, message compression when appending has been fixed.

Updates golang/go#16218

Change-Id: I3f42aa6e653e2990fa90368a2803e588ea15b85a
Reviewed-on: https://go-review.googlesource.com/97716Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>