More consistency to the global and user configuration setup.

* rename (again) the option to become: `opam config setup`
* split the options into the user ones (~/.ocamlinit, ~/.profile) and the global ones (load auto-completion and opam-switch-eval)
* meta options: `opam config setup --all`, `opam config setup --global` and `opam config --user`
* better error message when ~/.profile has already been updated by a different OPAM root
* add --dot-profile to `opam init` to select the default config file to update (default is ~/.profile)
* rename config options for `opam init` for setup: `--no-setup` and `--auto-setup`. Default behavior is asking the user.
`opam init --dot-profile=FILE` should pick as default config file given the one given by the user.