Should resolve the failure in
https://github.com/coredns/coredns/pull/2541.

This change is breaking to clustering plugin developers (not Caddy
users), but logical, since only the caddytls package uses CertMagic
directly (the httpserver package also uses it, but only because it also
uses the caddytls plugin); and it is early enough that no clustering
plugins really exist yet.

This will also require a change of devportal
so that it looks for a different registration function, which has moved
to the caddytls package.