Wireguard support for Mullvad and Windscribe (#565)

- `internal/wireguard` client package with unit tests
- Implementation works with kernel space or user space if unavailable
- `WIREGUARD_PRIVATE_KEY`
- `WIREGUARD_ADDRESS`
- `WIREGUARD_PRESHARED_KEY`
- `WIREGUARD_PORT`
- `internal/netlink` package used by `internal/wireguard`

internal/wireguard/constructor.go

@@ -0,0 +1,30 @@
+package wireguard
+
+import "github.com/qdm12/gluetun/internal/netlink"
+
+var _ Wireguarder = (*Wireguard)(nil)
+
+type Wireguarder interface {
+	Runner
+	Runner
+}
+
+type Wireguard struct {
+	logger   Logger
+	settings Settings
+	netlink  netlink.NetLinker
+}
+
+func New(settings Settings, netlink NetLinker,
+	logger Logger) (w *Wireguard, err error) {
+	settings.SetDefaults()
+	if err := settings.Check(); err != nil {
+		return nil, err
+	}
+
+	return &Wireguard{
+		logger:   logger,
+		settings: settings,
+		netlink:  netlink,
+	}, nil
+}