This commit implements MAC cloning feature in NetworkManager. To support that,
'PermHwAddress' property is added into *.Device.Wired and *.Device.Wireless
interfaces. The permanent MAC address is obtained when creating the device, and
is used for 'locking' connections to the device. If a cloned MAC is specified
in connection to be activated, the MAC is set to the interface in stage1. While
disconecting, the permanent MAC is set back to the interface.
Since a link-local address should pretty much always be
assigned to an active interface, avoid flushing the routes
that allow link-local communication to work.
Use the interfaces kernel index when we can to avoid unecessary
iface->index lookups; and let callers figure out which address
family they really want to flush.
Where we can do so, let's use ifindex since that's actually unique
and doesn't change when the interface name changes. We already use
ifindex in a bunch of places, and netlink *only* uses ifindex, so
this will make it easier later when we move over to ifindexes fully.
