parse existing zone file records AKA non-destructive zone file edition

If we want to support migrations into aegir of external domains, we'll want to be able to parse existing zonefiles, so we'll at least need to merge in the zone records into the configuration when loading a zonefile.

This way we may even be able to allow external tools to modify zonefiles directly, although that requirement is strongly diminished by the clean provision-zone interface right now. If we don't allow modification of zonefiles, we should explicitely state that in the zonefile template.

But at least here we should be able to import external zonefiles, in batch. I had a bind zonefile parser back then but it was scrapped in the 4th rewrite of the DNS code.

Ideally, we could even ditch the PHP storage for zones if we are able to reliable read and write them.