Register an already present local switch globally

[rjbou]

For the moment only ways to register an already present local switch in the global config is:

• opam switch . that fails but update global config
• opam update -u
• opam clean -s

As we need to a write lock to write the config file, and a switch load to launch fixing switch list, these commands give such a combination.
We should have something more user-friendly? or at least writes it in the FAQ for users that don't want to manually edit their config file.