change /usr/share/mysql to /usr/share/mariadb

To facilitate the removal of mysql naming in MariaDB products changing this directory name is part of it.

These appear to be largely internally used covering

• mysql-test (rename to mariadb-test at same time?)
• minibenchmark
• translations of error messages
• install/ upgrade scripts used by mariadb-install-db
• example files
• galera/mroonga/apparmor/selinux/systemd files
• /usr/share/mysql/mysql.init