A firewall pair's CARP addresses are a third virtual IP mechanism. Unlike
vip-manager and keepalived, a CARP reading says who holds the address: a
firewall's interface list need not show it, but CARP's state names the MASTER.
- VirtualIpsDict gains carp (NotRequired), VirtualIpDict gains holds
(NotRequired; None leaves the holder to the interfaces).
- carp_virtual_ip() shapes one address bound to a VHID: MASTER holds, BACKUP
stands by, INIT and DISABLED neither run nor hold, an unknown state says
nothing. The same for OPNsense and pfSense; a driver supplies the fields.
netOrk #1121.
christianmanivong