The basic process is to send a routing socket announcement that the
interface has departed, change if_xname, update the sockaddr_dl
associated with the interface, and announce the arrival of the interface
on the routing socket.
As part of this change, ifunit() is greatly simplified by testing
if_xname directly. if_clone_destroy() now uses if_dname to look up the
cloner for the interface and if_dunit to identify the unit number.
Reviewed by: ru, sam (concept)
Vincent Jardin <vjardin AT free.fr>
Max Laier <max AT love2party.net>
4f14e0fa86