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/configuration/selection.go

@@ -4,12 +4,13 @@ import (
 	"fmt"
 	"net"
 
+	"github.com/qdm12/gluetun/internal/constants"
 	"github.com/qdm12/golibs/params"
 )
 
 type ServerSelection struct { //nolint:maligned
 	// Common
-	VPN      string `json:"vpn"`
+	VPN      string `json:"vpn"` // note: this is required
 	TargetIP net.IP `json:"target_ip,omitempty"`
 	// TODO comments
 	// Cyberghost, PIA, Protonvpn, Surfshark, Windscribe, Vyprvpn, NordVPN
@@ -39,7 +40,8 @@ type ServerSelection struct { //nolint:maligned
 	// VPNUnlimited
 	StreamOnly bool `json:"stream_only"`
 
-	OpenVPN OpenVPNSelection `json:"openvpn"`
+	OpenVPN   OpenVPNSelection   `json:"openvpn"`
+	Wireguard WireguardSelection `json:"wireguard"`
 }
 
 func (selection ServerSelection) toLines() (lines []string) {
@@ -91,7 +93,11 @@ func (selection ServerSelection) toLines() (lines []string) {
 		lines = append(lines, lastIndent+"Numbers: "+commaJoin(numbersString))
 	}
 
-	lines = append(lines, selection.OpenVPN.lines()...)
+	if selection.VPN == constants.OpenVPN {
+		lines = append(lines, selection.OpenVPN.lines()...)
+	} else { // wireguard
+		lines = append(lines, selection.Wireguard.lines()...)
+	}
 
 	return lines
 }
@@ -137,6 +143,20 @@ func (settings *OpenVPNSelection) readProtocolAndPort(env params.Env) (err error
 	return nil
 }
 
+type WireguardSelection struct {
+	CustomPort uint16 `json:"custom_port"` // Mullvad
+}
+
+func (settings *WireguardSelection) lines() (lines []string) {
+	lines = append(lines, lastIndent+"Wireguard selection:")
+
+	if settings.CustomPort != 0 {
+		lines = append(lines, indent+lastIndent+"Custom port: "+fmt.Sprint(settings.CustomPort))
+	}
+
+	return lines
+}
+
 // PortForwarding contains settings for port forwarding.
 type PortForwarding struct {
 	Enabled  bool   `json:"enabled"`