wrrup: web RR update

A standards-based web user interface for editing DNS zones.

This program talks to the name server using normal DNS queries to read
zone data, and edits the data using RFC 2136 DNS UPDATE messages
authenticated using RFC 2845 TSIG.

No configuration is requireed: given a zone name, the master name
server address is obtained from the DNS, usernames and passwords
(i.e. TSIG owner names and secrets) are configured on the name server,
and HTTP authentication is gatewayed directly to TSIG authentication.

The code has minimal built-in knowledge of RR types, instead relying
on draft-levine-dnsextlang RDATA format descriptions.

It does not provide any facilities for editing the name server
configuration or zone metadata such as keys.