The IPv4 code should clean up multicast group state when an interface
goes away. Without this change, it leaks in_multi (and often ether_multi
state) if many clonable interfaces are created and destroyed in quick
succession.

The concept of this fix is borrowed from KAME. Detailed information about
this behaviour, as well as test cases, are available in the PR.

PR: kern/78227
MFC after: 1 week