FR: Super read-only mode

In MDEV-6039, serg says that super read-only looks fb specific. It isn't.

Feature does: Super read-only works when regular read-only itself is not enough, so this feature prevents writes by SUPER users too.

It is implemented in Percona Server 5.6 (https://www.percona.com/doc/percona-server/5.6/management/super_read_only.html) and also in MySQL 5.7 (https://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-8.html). Documented: https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_super_read_only and WL#6799 - https://dev.mysql.com/worklog/task/?id=6799

Mailing list post about it where serg talks about this: https://lists.launchpad.net/maria-developers/msg08911.html