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/vpn/wireguard.go

@@ -0,0 +1,45 @@
+package vpn
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"github.com/qdm12/gluetun/internal/configuration"
+	"github.com/qdm12/gluetun/internal/firewall"
+	"github.com/qdm12/gluetun/internal/netlink"
+	"github.com/qdm12/gluetun/internal/provider"
+	"github.com/qdm12/gluetun/internal/provider/utils"
+	"github.com/qdm12/gluetun/internal/wireguard"
+)
+
+var (
+	errGetServer       = errors.New("failed finding a VPN server")
+	errCreateWireguard = errors.New("failed creating Wireguard")
+)
+
+// setupWireguard sets Wireguard up using the configurators and settings given.
+// It returns a serverName for port forwarding (PIA) and an error if it fails.
+func setupWireguard(ctx context.Context, netlinker netlink.NetLinker,
+	fw firewall.VPNConnectionSetter, providerConf provider.Provider,
+	settings configuration.VPN, logger wireguard.Logger) (
+	wireguarder wireguard.Wireguarder, serverName string, err error) {
+	connection, err := providerConf.GetConnection(settings.Provider.ServerSelection)
+	if err != nil {
+		return nil, "", fmt.Errorf("%w: %s", errGetServer, err)
+	}
+
+	wireguardSettings := utils.BuildWireguardSettings(connection, settings.Wireguard)
+
+	wireguarder, err = wireguard.New(wireguardSettings, netlinker, logger)
+	if err != nil {
+		return nil, "", fmt.Errorf("%w: %s", errCreateWireguard, err)
+	}
+
+	err = fw.SetVPNConnection(ctx, connection, settings.Wireguard.Interface)
+	if err != nil {
+		return nil, "", fmt.Errorf("%w: %s", errFirewall, err)
+	}
+
+	return wireguarder, connection.Hostname, nil
+}