Add the ability to add a record to a zone if the name is not already in use in the zone.

This is to allow a machines to grab a unused name for themselves. Return REFUSED if the name already exists in the zone unless the record itself already exists. Normally the type list would be restricted to KEY but other types could be used.

This would normally be used in conjunction with self/self-sub updates for further updates.

I would restrict this to a single label below given names to prevent one machine hijacking the namespace of another.