Trigger topo change on every link up after link down

[wryun]

This causes DHCP renewals to happen when you disconnect/reconnect cables or re-establish wifi connections on a bridge.

I suspect I haven't done this in the right way - i.e. there's some implication I'm missing from causing TOPO_CHANGE all the time, though the 'auth' code does it anyway. But it's very useful for us in two situations:

• if we have an AP device (i.e. bridging ethernet and AP), then it means if you disconnect/reconnect the ethernet link it will request a new address (behaving in the way people would expect).
• if we have an Extender style device (i.e. bridging ethernet and STA), then if the STA link goes does it re-requests. Why is this useful? Because the AP might have reconfigured itself in some way; in our case, switching from a router (forwarding from AP->WAN) to an AP with ethernet bridged, in which case the DHCP server available will have changed. It's also useful in the case that the AP has reset if the AP is a DHCP server, in which case its DHCP lease table will have been cleared so it's nice from a UI perspective to have the STA renew.