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/tun_args.go

@@ -0,0 +1,6 @@
+package iface
+
+type MobileIFaceArguments struct {
+	TunAdapter TunAdapter // only for Android
+	TunFd      int        // only for iOS
+}