ip6_mroute: Handle interface detach events

[markj]

Summary

When an interface goes away we need to make sure the v6 multicast
routing tables don't carry any dangling references to the ifnet. The v4
code handles this already. Copy the approach there and use an
eventhandler to purge the corresponding MIF, if one exists, and further
go through all routes in the FIB and remove references to the interface.