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_link_linux.go

@@ -0,0 +1,33 @@
+//go:build linux && !android
+
+package iface
+
+import "github.com/vishvananda/netlink"
+
+type wgLink struct {
+	attrs *netlink.LinkAttrs
+}
+
+func newWGLink(name string) *wgLink {
+	attrs := netlink.NewLinkAttrs()
+	attrs.Name = name
+
+	return &wgLink{
+		attrs: &attrs,
+	}
+}
+
+// Attrs returns the Wireguard's default attributes
+func (l *wgLink) Attrs() *netlink.LinkAttrs {
+	return l.attrs
+}
+
+// Type returns the interface type
+func (l *wgLink) Type() string {
+	return "wireguard"
+}
+
+// Close deletes the link interface
+func (l *wgLink) Close() error {
+	return netlink.LinkDel(l)
+}