Netstack (#1403)

Add netstack support for the agent to run it without privileges.

- use interface for tun device
- use common IPC for userspace WireGuard integration
- move udpmux creation and sharedsock to tun layer

iface/uapi_windows.go

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