isc-dhcp: fix startup issues with RFC-1918 PTR RRs

isc-dhcp: fix startup issues with RFC-1918 PTR RRs

If isc-dhcp gets restarted, it might have to deal with RFC-1918 zones
being previously populated by an earlier instance.  In that case, we
need to know if we're modified versus initially adding the zones.

The special handling of RFC-1918 zones in Bind is quirky, and there
should be a patch soon to make it more friendly, but in the meantime
you might have to use:

disable-empty-zone 168.192.in-addr.arpa;

Or similar depending on which address block you poach.

Signed-off-by: Philip Prindeville <redacted>