message: nydusd: make thier names consistent in singleton mode
  We have already rename subcommand from daemon to singleton
  We'd better to rename the related functions and log messages.
diff: @@ -705,14 +705,14 @@ fn process_default_fs_service(
  ));
  }
  -fn process_daemon_arguments(
  +fn process_singleton_arguments(
  subargs: &SubCmdArgs,
  _apisock: Option<&str>,
  bti: BuildTimeInfo,
  ) -> Result<()> {
  - info!("Start Nydus in daemon mode!");
  + info!("Start Nydus in singleton mode!");
  let daemon = create_daemon(subargs, bti).map_err(|e| {
  - error!("Failed to start daemon: {}", e);
  + error!("Failed to start singleton daemon: {}", e);
  e
  })?;
  DAEMON_CONTROLLER.set_singleton_mode(true);
  @@ -744,7 +744,7 @@ fn main() -> Result<()> {
  // Safe to unwrap because the subcommand is `singleton`.
  let subargs = args.subcommand_matches("singleton").unwrap();
  let subargs = SubCmdArgs::new(&args, subargs);
  - process_daemon_arguments(&subargs, apisock, bti)?;
  + process_singleton_arguments(&subargs, apisock, bti)?;
  }
  Some("fuse") => {
  // Safe to unwrap because the subcommand is `fuse`.
  