crypt: change backend encode/decode to output a plain list

This commit changes the output of the rclone backend encode crypt: and
decode commands to output a plain list of decoded or encoded file
names.

This makes the command much more useful for command line scripting.

backend/crypt/crypt.go

@@ -705,7 +705,8 @@ var commandHelp = []fs.CommandHelp{
 	{
 		Name:  "encode",
 		Short: "Encode the given filename(s)",
-		Long: `Encode the given filename(s)
+		Long: `This encodes the filenames given as arguments returning a list of
+strings of the encoded results.
 
 Usage Example:
 
@@ -716,7 +717,9 @@ var commandHelp = []fs.CommandHelp{
 	{
 		Name:  "decode",
 		Short: "Decode the given filename(s)",
-		Long: `Decode the given filename(s)
+		Long: `This decodes the filenames given as arguments returning a list of
+strings of the decoded results. It will return an error if any of the
+inputs are invalid.
 
 Usage Example:
 
@@ -738,20 +741,20 @@ var commandHelp = []fs.CommandHelp{
 func (f *Fs) Command(ctx context.Context, name string, arg []string, opt map[string]string) (out interface{}, err error) {
 	switch name {
 	case "decode":
-		out := make(map[string]string)
+		out := make([]string, 0, len(arg))
 		for _, encryptedFileName := range arg {
 			fileName, err := f.DecryptFileName(encryptedFileName)
 			if err != nil {
 				return out, errors.Wrap(err, fmt.Sprintf("Failed to decrypt : %s", encryptedFileName))
 			}
-			out[encryptedFileName] = fileName
+			out = append(out, fileName)
 		}
 		return out, nil
 	case "encode":
-		out := make(map[string]string)
+		out := make([]string, 0, len(arg))
 		for _, fileName := range arg {
 			encryptedFileName := f.EncryptFileName(fileName)
-			out[fileName] = encryptedFileName
+			out = append(out, encryptedFileName)
 		}
 		return out, nil
 	default: