Ipv6 Is Not A New Thing

Most is said in the commits. The basic idea is to reuse the (already provided) structures and streamline in the xfr-handler and server/zone structures in preparation for the zone events.

Typo: prefix not address. Why is the netblock_t struct not used, btw?

Typo fixed in 09cb0f49, netblock_t is not used because I think it's convenient to pass address and prefix separately and use netblock_t internally.

Otherwise OK.