Add support for VRRP unicast

This patch adds the ability to use VRRP over unicast. Unicast IP addresses may be specified for
each VRRP instance with the 'unicast_peer' configuration keyword. When a VRRP instance has one
or more unicast IP address defined, VRRP advertisements will be sent to each of those
addresses. Unicast IP addresses may be either IPv4 or IPv6.

The original idea comes from Willy Tarreau. Ryan O'Hara extended the concept to support
multiple peers as well as IPv6 unicast peer.

If you are planing to use this option, ensure every ip addresses present in unicast_peer
configuration block do not belong to the same router/box. Otherwise it will generate duplicate
packet at reception point.