Previously, we only accepted a single string for TXT records and then
chunked it into 255 byte segments. This is wrong. TXT records are a
sequence of strings, each up to 255 bytes.

Updates golang/go#24288

Change-Id: Ib2c085ec127ccecf0c7bda930100b667cabc1f4b
Reviewed-on: https://go-review.googlesource.com/100196Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>