MDEV-11170: MariaDB 10.2 cannot start on MySQL 5.7 datadir:

Fatal error: mysql.user table is damaged or in unsupported 3.20 format

The problem stems from MySQL 5.7.6. According to MySQL documentation:
In MySQL 5.7.6, the Password column was removed and all credentials are
stored in the authentication_string column.

If opening a MySQL 5.7.6 (and up) datadir with MariaDB 10.2, the user table
appears corrupted. In order to fix this, the server must be started with
--skip-grant-tables and then a subsequent mysql_upgrade command must be
issued.

This patch updates the mysql_upgrade command to also add the removed
Password column. The password column is necessary, otherwise
the mysql_upgrade script fails due to the Event_scheduler not being able
to start, as it can't find Event_priv in the table where it ought to be.
MySQL's version has column position 28 (0 index) vs our datadir version
expects position 29.

scripts/mysql_system_tables_fix.sql

@@ -164,6 +164,12 @@ ALTER TABLE user
   MODIFY Host char(60) NOT NULL default '',
   MODIFY User char(80) NOT NULL default '',
   ENGINE=MyISAM, CONVERT TO CHARACTER SET utf8 COLLATE utf8_bin;
+
+# In MySQL 5.7.6 the Password column is removed. Recreate it to preserve the number
+# of columns MariaDB expects in the user table.
+ALTER TABLE user
+  ADD Password char(41) character set latin1 collate latin1_bin NOT NULL default '' AFTER User;
+
 ALTER TABLE user
   MODIFY Password char(41) character set latin1 collate latin1_bin NOT NULL default '',
   MODIFY Select_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL,