isc-dhcp: detect whether ddns transfers are IPv4 only

isc-dhcp: detect whether ddns transfers are IPv4 only

If named is configured to not listen on any IPv6 interfaces,
then we should run 'nsupdate' with the '-4' argument.

Also:

* cleanup RFC-1918 address detection;
* don't generate PTR records for domain entries that aren't
  RFC-1918 addresses or these will generate NOTAUTH failures;

We're assuming that we're doing DNS split-horizon and that
internal addresses aren't routable.

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>