zone: New record type :svc creates A records without PTR records.

zone: New record type :svc creates A records without PTR records.

The :svc record type inserts an A record into the zone without inserting
a matching PTR record into the reverse zone(s).  This is useful for
service role addresses.

To make this work, we add a new slot make-ptr-p to zone records which is
used to label those A records which are eligible to be turned into
PTRs.  This mechanism is, I hope, considerably simpler than the old
defsubp system.