We don't want mirror.pkgbuild.com's DNS server to be a
single-point-of-failure, so this commit adds multiple authoritative DNS
servers for the zone. The extra DNS servers are run on the geomirror
servers.

The _acme-challenge zone, used for obtaining certificates, is run solely
on mirror.pkgbuild.com's DNS server, to avoid syncing DNS records
between the servers (KISS).