Make mysql-optimize.cron and mysql-backup.cron handle missing `mysql` + `mysqldump` + `mysqlcheck`

These commands were symlinks to mariadb, mariadb-dump and mariadbcheck respectively. In MariaDB 11.0, these commands are deprecated, so these symlinks were moved to a separate 'compat' Debian package. By letting tuxis-cron fall back to the old commands, we don't need to install this compat package, while also handling cases where MySQL is used instead of MariaDB.