sys schema from good-dba does not work with MariaDB

[ebroto]

The sys schema suggested for MariaDB in the README does not work out of the box. It's incomplete and makes mysqltuner fail:

-------- Performance schema: Top 5 user per connection ---------------------------------------------
[!!] failed to execute: select user, total_connections from sys.user_summary order by total_connections desc LIMIT 5
[!!] FAIL Execute SQL / return code: 256
[--] No information found or indicators deactivated.

FromDual seems to have a working for in https://github.com/FromDual/mariadb-sys . I've tested it successfully against MariaDB 10.3.