Fix race conditions involved in setting IP multicast options.  This should fix Dennis Fortin's problem for good, if I've got it figured out right.
Fix race conditions involved in setting IP multicast options.  This should
fix Dennis Fortin's problem for good, if I've got it figured out right.

(The problem was that a `struct ifaddr' could get deleted out from under
the current requester, thus leaving him with an invalid interface pointer
and causing even more bogus accesses.)

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