Merge branch 'zone_api-zone_update' into 'master'

This is the initial merge request for zone-api. Changes are quite
simple, mainly moving code from one place to the other and adding
a bunch of functions to deal with it. So far the only advantage is
a bit simpler code in the DDNS code and simpler API for it. The insides
are not so nice yet as it requires further work on the changesets.