Use cmdliner in all bin/ programs

[rgrinberg]

Or at the very least use cmdliner in the ones that are meant for public consumption. E.g.:

https://github.com/mirage/ocaml-cohttp/blob/master/bin/cohttp_server_lwt.ml#L163

I'd also prefer to use cmdliner over core's command even in the async backend. Because:

• cmdliner is better
• It's more uniform this way

[avsm]

I agree with all of that except for the Async backend. If Command's the canonical CLI parser there, it's best to stick to that stdlib's conventions...