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/netstack/dialer.go

@@ -0,0 +1,32 @@
+package netstack
+
+import (
+	"context"
+	"net"
+
+	log "github.com/sirupsen/logrus"
+	"golang.zx2c4.com/wireguard/tun/netstack"
+)
+
+type Dialer interface {
+	Dial(ctx context.Context, network, addr string) (net.Conn, error)
+}
+
+type NSDialer struct {
+	net *netstack.Net
+}
+
+func NewNSDialer(net *netstack.Net) *NSDialer {
+	return &NSDialer{
+		net: net,
+	}
+}
+
+func (d *NSDialer) Dial(ctx context.Context, network, addr string) (net.Conn, error) {
+	log.Debugf("dialing %s %s", network, addr)
+	conn, err := d.net.Dial(network, addr)
+	if err != nil {
+		log.Debugf("failed to deal connection: %s", err)
+	}
+	return conn, err
+}