Remove mkludge stuffs

For unknown reasons, IPv6 multicast addresses are linked to a first
IPv6 address assigned to an interface. Due to the design, when removing
a first address having multicast addresses, we need to save them to
somewhere and later restore them once a new IPv6 address is activated.
mkludge stuffs support the operations.

This change links multicast addresses to an interface directly and
throws the kludge away.

Note that as usual some obsolete member variables remain for kvm(3)
users. And also sysctl net.inet6.multicast_kludge remains to avoid
breaking old ifmcstat.

TODO: currently ifnet has a list of in6_multi but obviously the list
should be protocol independent. Provide a common structure (if_multi
or something) to handle in6_multi and in_multi together as well as
ifaddr does for in_ifaddr and in6_ifaddr.

(ozaki-r)