EINVAL. This has been confirmed by Bill Fenner. The wrong socket led to
masses of syslog entries on hosts connected to the mbone and cache entries
deleted to early.
XXX As the socket need to be the one that is ip_mrouter in netinet/ip_mroute.c,
XXX the kernel could be modified to always return the data for ip_mrouter.
XXX Bill Fenner suggests to upgrade to 3.9.beta-3 with -DIOCTL_OK_ON_RAW_SOCKET
