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).

We had a GeoIP mirror in the past based on nginx and its GeoIP module,
but it didn't perform very well, due to the high latency (asking a
central server for the package and then redirected to the closest
mirror).

One of the reasons for offering this service, is so we can relieve
mirror.pkgbuild.com which is burning a ton of traffic (50TB/month),
likely due to it being the default mirror in our Docker image. Another
reason is so we can offer a link to our arch-boxes images in libosinfo
(used by gnome-boxes, virt-install and virt-manager), with good enough
performance for most users.

This time we take a different approach and use a DNS based solution,
which means the latency penalty is only paid once (the first DNS
request). The downside is that the mirrors must have a valid certificate
for the same domain name, which makes using third-party mirrors a
challenge. So for now, we are just using the sponsored mirorrs
controlled by the DevOps team.

Fix #101