[client] Refactor/iface pkg (#2646)

Refactor the flat code structure

client/iface/configurer/uapi_windows.go

@@ -0,0 +1,11 @@
+package configurer
+
+import (
+	"net"
+
+	"golang.zx2c4.com/wireguard/ipc"
+)
+
+func openUAPI(deviceName string) (net.Listener, error) {
+	return ipc.UAPIListen(deviceName)
+}