dlna: simplify search method for associating subtitles with media nodes

Seems to be some corner cases that are not being handled, so taking a different
approach that should be a little more robust.

Also, changing resources to be served under a subpath:  We've been serving
media at /res?path=%2Fdir%2Ffilename.mp4; change that to be just /r/dir/filename.mp4.
It's cleaner, easier to reason about, and a necessary first step towards just
serving the resources via httplib anyway.

cmd/serve/dlna/dlna_util.go

@@ -218,3 +218,14 @@ func serveError(what interface{}, w http.ResponseWriter, text string, err error)
 	fs.Errorf(what, "%s: %v", text, err)
 	http.Error(w, text+".", http.StatusInternalServerError)
 }
+
+// Splits a path into (root, ext) such that root + ext == path, and ext is empty
+// or begins with a period.  Extended version of path.Ext().
+func splitExt(path string) (string, string) {
+	for i := len(path) - 1; i >= 0 && path[i] != '/'; i-- {
+		if path[i] == '.' {
+			return path[:i], path[i:]
+		}
+	}
+	return path, ""
+}