Allow OS root user access to the REST-API

During an upgrade of MaxScale, the upgrade process must signal the running MaxScale that an upgrade is starting and that it should stop accepting new connections once the new process has started. It also can be used to prevent the creation of new objects which is needed to prevent newer versions of the modules from being accidentally loaded into the older version of MaxScale.

This would also improve the security of the installation by only allowing the OS root user to communicate with MaxScale by default, similar to how most MariaDB installations are set up nowadays to use UNIX domain socket authentication.