Dropping --migrate-backend option and adding --add-user option to the core.

Dropping --migrate-backend option and adding --add-user option to the core.
--select-backend supersedes --migrate-backend now as it switches your
backend, merges the old to thew new backend (if supported by both
backends and the new backend was found uninitialized). Otherwise
prompt for credentials to add a new user.
You can now also use the --add-user option at any time to add a core user.