Add the kernel side of network interface renaming support.

Add the kernel side of network interface renaming support.

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>