dns: allow PTR records in arbitrary zones
PTR records in zones other than in-addr.arpa and in6.arpa are legal,
e.g. DNS-SD [RFC6763] uses such records. If in a reverse zone
proceed with the existing checks, otherwise just accept the record.
Fixes: https://pagure.io/freeipa/issue/5566
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>