Fix location estimation on Windows

Also, switch to a better Portal Test IP.

netenv/location_default.go

@@ -0,0 +1,9 @@
+//+build !windows
+
+package netenv
+
+import "net"
+
+func newICMPListener(_ string) (net.PacketConn, error) {
+	return net.ListenPacket("ip4:icmp", "0.0.0.0")
+}