Make opam init easier to use

[kit-ty-kate]

I personally find some of the options of opam init confusing. In particular:

• opam init --reinit: I would expect this to be the default behaviour of opam init. Nobody can remember that opam init doesn't actually do anything (the command doesn't tell you that) and they should instead add --reinit for upgrades (I myself can't remember it)
• It would be nice if opam init on a bare opam root were to initialize a default switch the same way opam init does without an opam root directory. For instance in https://github.com/ocaml/opam/issues/4820#issuecomment-909104805 it was recommended to use rm -rf ~/.opam which in my opinion shouldn't be something we should recommend (if we had the tools to avoid it, which isn't the case at the moment). In this particular case, if we want opam init to do that, it would be nice for opam install ... to fail with run opam init or opam switch create or something of the sort to advertise the opam init command.

[kit-ty-kate]

See https://discuss.ocaml.org/t/ocaml-with-emacs-on-win10/8602 for a fresh example of the struggle for users wrt this issue