bug#49596: 28.0.50; shadowfile.el: `shadow-define-literal-group' is broken

[Max Brieiev]

Hi Michael,

Sorry for being quiet for a long time.
Yes, the issue is fixed.

However, the interface of `shadow-define-literal-group` is somewhat
obscure:

- `M-x shadow-define-literal-group RET RET` will insert `nil` into
  `shadow-literal-groups`, which is probably not correct?

- The first prompt `Host identification or cluster name: ` isn't very
  descriptive. There is not much docs and I had to learn by trial and
  error that it accepts either tramp format like `/ssh:user@host:` for
  remote endpoints or a plain string like `my-linux-box` for local host
  names. And it seems if it can't resolve provided host name it will
  silently insert `nil` into `shadow-literal-groups`, which also doesn't
  seem correct.

That said, it is easier to set `shadow-literal-groups` manually, because
you can manually test accessibility of remote files and then simply
append the resulting tramp file name into the variable.