The "users" entry is now a dictionary mapping user names to
passwords and permissions.  In order to allow for wildcards,
there is a new type of password, the wildcard password, and
an extra array called "fallback-users".

The field "allow-anonymous" no longer exists, this is now
the default behaviour.  The field "allow-subgroups" has been
renamed to "auto-subgroups".

We provide backwards compatibility for group definition files,
but not for the config.json file, where the old "admin" array
is simply ignored.