GitLab

The reason for this fix was that when I tried to run mysql_upgrade
at home to update an old 10.5 installation, mysql_upgrade failed
with warnings about mariadb.sys user not existing.

If the server was started with --skip-grants, there would be no warnings
from mysql_upgrade, but in some cases running mysql_upgrade again could
produce new warnings.

The reason for the warnings was that any access of the mysql.user view
will produce a warning if the mariadb.sys user does not exists.

Fixed with the following changes:
- Disable warnings about mariadb.sys user not existing
- Don't overwrite old mariadb.sys entries in tables_priv and global_priv
- Ensure that tables_priv has an entry for mariadb.sys if the user exists.
  This fixes an issue that tables_priv would not be updated if there
  was a failure directly after global_priv was updated.