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/log.go

@@ -0,0 +1,26 @@
+package wireguard
+
+import (
+	"fmt"
+
+	"golang.zx2c4.com/wireguard/device"
+)
+
+//go:generate mockgen -destination=log_mock_test.go -package wireguard . Logger
+
+type Logger interface {
+	Debug(s string)
+	Info(s string)
+	Error(s string)
+}
+
+func makeDeviceLogger(logger Logger) (deviceLogger *device.Logger) {
+	return &device.Logger{
+		Verbosef: func(format string, args ...interface{}) {
+			logger.Debug(fmt.Sprintf(format, args...))
+		},
+		Errorf: func(format string, args ...interface{}) {
+			logger.Error(fmt.Sprintf(format, args...))
+		},
+	}
+}