vrrp: IPv4 & IPv6 multicast group tweaking

Meno Abels extended vrrp framework to support customized
multicast addresses. The address could be set for ipv4 and ipv6
in the global_defs config section using the keywords
vrrp_mcast_group4 and vrrp_mcast_group6.

There are some stupid switches which does a special processing
to 224.0.0.0/8 multicast packets which causes packets drop from
queue overflows in environments which creates 100 and more
multicast control plane packets a second.