Now it works both from the command line and from emacs.
It's ok for me that the the flag is -n.
I think that the arc script was named "arc.sh" because the folder which is used by the news server is "arc", and it would conflict. We can either take back the "arc.sh" name or change the news server's directory to something else, perhaps "www" like in the master branch.
Edit: We should also change the flag of the default program name in inferior-arc.el (line 95):