distorted.lisp: Better processing of anycast addresses.

Introduce a custom `:anycast' record parser which hacks on descriptions
of which address families provide which services and does the right
thing.

This stuff is complicated because IPv6 anycast addresses actually
work globally so we might as well publish them properly.  Also, actual
anycast addresses have dedicated allocations, so it's right to maintain
PTR records for them; but the static-provider addresses are service
names and don't want reverse records.