Debian 9 (Stretch): MariaDB instead of MySQL, X without root

The release from June 17, 2017 swapped the default database implementation: the mysql-* packages gave way to MariaDB, which installs in their place. For existing systems that meant planning for a different server behind the same client name, not just a higher version number.

The second change is less visible and more important: on most setups the X server now starts without root privileges. The graphical layer stopped being a permanently running process with full rights – one whole piece of the system that used to be worth attacking for the highest privileges.