mariadb-install-db --auth-root-authentication-method=normal created 4
root accounts by default, but only two of these had PROXY privilege
granted.

mariadb-install-db (default option
--auth-root-authentication-method=socket) as non-root user also didn't
grant PROXY priv to the created nonroot@localhost user.

To fix this, in mysql_system_tables_data.sql, we re-use tmp_user_nopasswd
as this contains the list of all root users.

REPLACE INTO tmp_proxies_priv SELECT @current_hostname, IFNULL(@auth_root_socket, 'root')
creates the $user@$current_host but will not error if @auth_root_socket
is null. Note @current_hostname lines are filtered out with
--cross-bootstrap in mariadb-install-db so it was needed to include this
expression for consistency.

Like the existing mysql_system_tables.sql is used to create teh
$user@localhost proxies_priv.

Test cases roles.acl_statistics, perfschema,privilege_table_io depends on the number of proxy users.

After:

--auth-root-authentication-method=normal:

MariaDB [mysql]> select * from global_priv;
+-----------+-------------+--------------------------------------------------------------------------------------------------------------------------+
| Host      | User        | Priv                                                                                                                     |
+-----------+-------------+--------------------------------------------------------------------------------------------------------------------------+
| localhost | mariadb.sys | {"access":0,"plugin":"mysql_native_password","authentication_string":"","account_locked":true,"password_last_changed":0} |
| localhost | root        | {"access":18446744073709551615}                                                                                          |
| bark      | root        | {"access":18446744073709551615}                                                                                          |
| 127.0.0.1 | root        | {"access":18446744073709551615}                                                                                          |
| ::1       | root        | {"access":18446744073709551615}                                                                                          |
| localhost |             | {}                                                                                                                       |
| bark      |             | {}                                                                                                                       |
+-----------+-------------+--------------------------------------------------------------------------------------------------------------------------+
7 rows in set (0.001 sec)

MariaDB [mysql]> select * from proxies_priv;
+-----------+------+--------------+--------------+------------+---------+---------------------+
| Host      | User | Proxied_host | Proxied_user | With_grant | Grantor | Timestamp           |
+-----------+------+--------------+--------------+------------+---------+---------------------+
| localhost | root |              |              |          1 |         | 2023-07-10 12:12:24 |
| 127.0.0.1 | root |              |              |          1 |         | 2023-07-10 12:12:24 |
| ::1       | root |              |              |          1 |         | 2023-07-10 12:12:24 |
| bark      | root |              |              |          1 |         | 2023-07-10 12:12:24 |
+-----------+------+--------------+--------------+------------+---------+---------------------+

--auth-root-authentication-method=socket:

MariaDB [mysql]> select * from proxies_priv;
+-----------+------+--------------+--------------+------------+---------+---------------------+
| Host      | User | Proxied_host | Proxied_user | With_grant | Grantor | Timestamp           |
+-----------+------+--------------+--------------+------------+---------+---------------------+
| localhost | root |              |              |          1 |         | 2023-07-10 12:11:55 |
| localhost | dan  |              |              |          1 |         | 2023-07-10 12:11:55 |
| bark      | dan  |              |              |          1 |         | 2023-07-10 12:11:55 |
+-----------+------+--------------+--------------+------------+---------+---------------------+
3 rows in set (0.017 sec)

MariaDB [mysql]> select * from global_priv;
+-----------+-------------+--------------------------------------------------------------------------------------------------------------------------------------------+
| Host      | User        | Priv                                                                                                                                       |
+-----------+-------------+--------------------------------------------------------------------------------------------------------------------------------------------+
| localhost | mariadb.sys | {"access":0,"plugin":"mysql_native_password","authentication_string":"","account_locked":true,"password_last_changed":0}                   |
| localhost | root        | {"access":18446744073709551615,"plugin":"mysql_native_password","authentication_string":"invalid","auth_or":[{},{"plugin":"unix_socket"}]} |
| localhost | dan         | {"access":18446744073709551615,"plugin":"mysql_native_password","authentication_string":"invalid","auth_or":[{},{"plugin":"unix_socket"}]} |
| localhost |             | {}                                                                                                                                         |
| bark      |             | {}                                                                                                                                         |
+-----------+-------------+--------------------------------------------------------------------------------------------------------------------------------------------+
5 rows in set (0.000 sec)

MariaDB [mysql]> show grants;
+----------------------------------------------------------------------------------------------------------------------------------------+
| Grants for dan@localhost                                                                                                               |
+----------------------------------------------------------------------------------------------------------------------------------------+
| GRANT ALL PRIVILEGES ON *.* TO `dan`@`localhost` IDENTIFIED VIA mysql_native_password USING 'invalid' OR unix_socket WITH GRANT OPTION |
| GRANT PROXY ON ''@'%' TO 'dan'@'localhost' WITH GRANT OPTION                                                                           |
+----------------------------------------------------------------------------------------------------------------------------------------+