Help mentions flags-subcommand even if it doesn't exists

[Axel Wagner]

I don't want a "flags" subcommand - if I have any top-level flags, I would rather they are all treated as "important". However, even if I don't add a flags subcommand or not, the help-subcommand always prints

Use "nbd flags" for a list of top-level flags

That shouldn't be printed, unless there is actually a flags subcommand.