opam remote set-url --add

[kit-ty-kate]

Similarly to git remote set-url --add this option would be useful to simplify things like opam remote add <name> <url> || opam remote set-url <name> <url> in scripts or other "steps to follow" used to make sure the recipient has the correct remote at the correct url.

As a side-note, opam remote add <name> <url> currently does not return an error when url is the current url of the repository name but does return an error when it's not, whereas git remote will raise an error in both cases.