- Reworked the code in dns_challenge.go to not assume nameserver is
port-less or defaults to 53. (messes up testing)
- Updated nameserver test to clear the fqdn2zone cache and return a dummy
SOA RR to make initial findZoneByFqdn call happy.
- Used publicsuffix library to determine if the "authorative" zone we found
is a public registry, in that case error out. (Also used by boulder btw)
Add a new environment variable RFC2136_TSIG_ALGORITHM that accepts the
TSIG algorithm pseudo-domain name. Let it default to
"hmac-md5.sig-alg.reg.int." if unset.
* new ChallengeProvider with Present and CleanUp methods
* new Challenge type describing `http-01`, `tls-sni-01`, `dns-01`
* new client.SetChallengeProvider to support custom implementations
