cmd: rclone selfupdate (#5080)

Implements self-update command
Fixes #548
Fixes #5076

cmd/cmd.go

@@ -548,6 +548,9 @@ func Main() {
 	setupRootCommand(Root)
 	AddBackendFlags()
 	if err := Root.Execute(); err != nil {
+		if strings.HasPrefix(err.Error(), "unknown command") {
+			Root.PrintErrf("You could use '%s selfupdate' to get latest features.\n\n", Root.CommandPath())
+		}
 		log.Fatalf("Fatal error: %v", err)
 	}
 }