internal/clientinfo: watch NDP table changes on Linux

So with clients which only use SLAAC, ctrld could see client's new ip as
soon as its state changes to REACHABLE.

Moreover, the NDP listener is also changed to listen on all possible
ipv6 link local interfaces. That would allow ctrld to get all NDP events
happening in local network.

SLAAC RFC: https://datatracker.ietf.org/doc/html/rfc4862

internal/clientinfo/client_info.go

@@ -224,6 +224,7 @@ func (t *Table) init() {
 			cancel()
 		}()
 		go t.ndp.listen(ctx)
+		go t.ndp.subscribe(ctx)
 	}
 	// PTR lookup.
 	if t.discoverPTR() {