dns/dnsmessage: implement fmt.GoStringer.GoString

This improves the debugability of the library and eases the transition
to this library. For example, test DNS messages defined with another DNS
library can be packed and then unpacked with this library. These
unpacked messages can have GoString called on them to generate new test
messages defined with this library.

Updates golang/go#16218

Change-Id: I602586500fd8202892ef04187d3bd8a11039cf27
Reviewed-on: https://go-review.googlesource.com/120697
Reviewed-by: Brad Fitzpatrick <[email protected]>
Run-TryBot: Brad Fitzpatrick <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: iangudger