in.c: when an interface address is deleted, keep its multicast membership
. records (attached to a struct multi_kludge) for attachment to the
. next address on the same interface. Also, in_multi structures now
. gain a reference to the ifaddr so that they won't point off into
. freed memory if an interface goes away and doesn't come back before
. the last socket reference drops. This is analogous to how it is
. done for routes, and seems to make the most sense.
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