serve dlna: sort the directory entries by directories first then alphabetically by name

Some media boxes don't sort the items returned from the DLNA server,
so sort them here, directories first then alphabetically by name.

See: https://forum.rclone.org/t/serve-dlna-files-order-directories-first/47790

cmd/serve/dlna/cds.go

@@ -11,6 +11,7 @@ import (
 	"path"
 	"path/filepath"
 	"regexp"
+	"sort"
 	"strings"
 
 	"github.com/anacrolix/dms/dlna"
@@ -158,6 +159,18 @@ func (cds *contentDirectoryService) readContainer(o object, host string) (ret []
 		}
 	}
 
+	// Sort the directory entries by directories first then alphabetically by name
+	sort.Slice(dirEntries, func(i, j int) bool {
+		iNode, jNode := dirEntries[i], dirEntries[j]
+		iIsDir, jIsDir := iNode.IsDir(), jNode.IsDir()
+		if iIsDir && !jIsDir {
+			return true
+		} else if !iIsDir && jIsDir {
+			return false
+		}
+		return strings.ToLower(iNode.Name()) < strings.ToLower(jNode.Name())
+	})
+
 	dirEntries, mediaResources := mediaWithResources(dirEntries)
 	for _, de := range dirEntries {
 		child := object{