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.

The mysql.user view password_expired column should display the right
result, in sync with whether an account has its password expired or not

For mariadb 10.4+ upgrades before this commit, the mysql.user view needs
to be dropped and recreated to actually make the view display the
correct value for the password_expired column.

don't GRANT UPDATE ON mysql.global_priv TO mariadb.sys@localhost;

Check if we really need the mariadb.sys user

Also fixes:
MDEV-21487: Implement option for mysql_upgrade that allows root@localhost to be replaced
MDEV-21486: Implement option for mysql_install_db that allows root@localhost to be replaced

Add user mariadb.sys to be definer of user view
(and has right on underlying table global_priv for
required operation over global_priv
(SELECT,UPDATE,DELETE))

Also changed definer of gis functions in case of creation,
but they work with any definer so upgrade script do not try
to push this change.