Added the facility to set the multicast address different to the
rfc specified.
The address could be set for ipv4 and ipv6 in the global_defs config
section using the keywords mcast_group4 and mcast_group6.
I need this of course 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.