Handle Thunderbolt network adapters

You may be interested in some changes I've made to the NetworkLink evidence source.

The current version did not play well with Thunderbolt network adapters.

When a Thunderbolt Ethernet adapter is unplugged, the interface disappears completely, rather than signalling that link has gone inactive, as is the case with built-in network interfaces. ControlPlane therefore never noticed that such an interface was unplugged.

I have now changed the method of enumerating interfaces to get the Network Services; these are all the interfaces listed in the locations in System Preferences and persists even if the interface is not currently available. With this method, I get reliable context switching on plugging/unpluggging of Thunderbolt network adapters.

There are some other minor tweaks, such as using GCD for async notifications.
Hope this will be useful.