net/tshttpproxy: add GetProxyForURL negative cache

Otherwise when PAC server is down, we log, and each log entry is a new
HTTP request (from logtail) and a new GetProxyForURL call, which again
logs, non-stop. This is also nicer to the WinHTTP service.

Then also hook up link change notifications to the cache to reset it
if there's a chance the network might work sooner.

wgengine/router/ifconfig_windows.go

@@ -242,7 +242,6 @@ func configureInterface(cfg *Config, tun *tun.NativeTun) error {
 		log.Printf("setPrivateNetwork: adapter %v not found after %d tries, giving up", guid, tries)
 	}()
 
-	routes := []winipcfg.RouteData{}
 	var firstGateway4 *net.IP
 	var firstGateway6 *net.IP
 	addresses := make([]*net.IPNet, len(cfg.LocalAddrs))
@@ -257,6 +256,7 @@ func configureInterface(cfg *Config, tun *tun.NativeTun) error {
 		}
 	}
 
+	var routes []winipcfg.RouteData
 	foundDefault4 := false
 	foundDefault6 := false
 	for _, route := range cfg.Routes {