feat(wireguard): add debug logs for IPv6 detection

- To debug issue #998
- Enable with `LOG_LEVEL=debug`

internal/wireguard/log.go

@@ -1,8 +1,6 @@
 package wireguard
 
 import (
-	"fmt"
-
 	"golang.zx2c4.com/wireguard/device"
 )
 
@@ -10,17 +8,15 @@ import (
 
 type Logger interface {
 	Debug(s string)
+	Debugf(format string, args ...interface{})
 	Info(s string)
 	Error(s string)
+	Errorf(format string, args ...interface{})
 }
 
 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...))
-		},
+		Verbosef: logger.Debugf,
+		Errorf:   logger.Errorf,
 	}
 }