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.
