* do not modify dns.Rcode when packing to wire format
When the message has an EDNS0 option in the additional section and
dns.Msg.Rcode is set to an extended rcode, dns.Msg.PackBuffer() modifies
dns.Msg.Rcode.
If you were to `Pack` the message and log it after, the Rcode would show
NOERROR.
Running the test before the change would error with:
```
=== RUN TestPackNoSideEffect
--- FAIL: TestPackNoSideEffect (0.00s)
msg_test.go:51: after pack: Rcode is expected to be BADVERS
```
after fixing dns.Msg.PackBuffer(), all tests are still passing.
Fixes#674
* address comments from PR#675
* limiting domain names to 255/63 octets/labels (#463)
(cherry picked from commit 0b729df06c)
* account for \ and \xxx in presentation format
(cherry picked from commit a094f774892fb4305051d185c2488cb43200c4d9)
* go fmt
* Add tests for UnpackDomainName
Domain names must not exceed 255 octets in wire format.
Ref gh-463
Ref gh-469
* Fix UnpackDomainName
* Introduce a long-domain sentinel error
A typed error would be better, but inconsistent with this library.
cf. https://dave.cheney.net/2016/04/27/dont-just-check-errors-handle-them-gracefully
chantra
Richard Gibson