message: nydusd: rename daemon subcommand to singleton
  So that it is clear that the subcommand means to run nydusd in a global
  single instance mode.
diff: @@ -324,8 +324,8 @@ fn append_virtiofs_subcmd_options(app: App<'static, 'static>) -> App<'static, 's
  }
  fn append_services_subcmd_options(app: App<'static, 'static>) -> App<'static, 'static> {
  - let subcmd = SubCommand::with_name("daemon")
  - .about("Run as a global daemon hosting multiple blobcache/fscache/virtiofs services.")
  + let subcmd = SubCommand::with_name("singleton")
  + .about("Run as a global daemon instance to service multiple blobcache/fscache/virtiofs services.")
  .arg(
  Arg::with_name("fscache")
  .long("fscache")
  