cmd/geth: support dumpconfig optionally saving to file (ethereum#18327)

* Changed dumpConfig function to optionally save to file

* Added O_TRUNC flag to file open and cleaned up code

cmd/geth/config.go

@@ -20,7 +20,6 @@ import (
 	"bufio"
 	"errors"
 	"fmt"
-	"io"
 	"math/big"
 	"os"
 	"reflect"
@@ -198,7 +197,17 @@ func dumpConfig(ctx *cli.Context) error {
 	if err != nil {
 		return err
 	}
-	io.WriteString(os.Stdout, comment)
-	os.Stdout.Write(out)
+
+	dump := os.Stdout
+	if ctx.NArg() > 0 {
+		dump, err = os.OpenFile(ctx.Args().Get(0), os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0644)
+		if err != nil {
+			return err
+		}
+		defer dump.Close()
+	}
+	dump.WriteString(comment)
+	dump.Write(out)
+
 	return nil
 }