Now that IGMP and MLD sysctls provide a clean API structures that do not
leak kernel internal stuff, reconnect ifmcstat(1) back to build. However,
disable kvm(3) support in it, since it requires uncovering tons of _KERNEL
defined declarations, which can be achieved either uncovering them globally
or providing dirty hacks such as _WANT_IFADDR. If anyone demands an
ifmcstat-like kvm-based tool, please take the code out of usr.sbin/ifmstat
and create a tool in src/tools/tools.