- Improve mysql_upgrade and add comments describing it's logic
- Don't look for mysql and mysqlcheck randomly, use dir where mysql_upgrade
was started from
- Don't look for mysql_fix_privilege_tables.sql randomly, compile
in the mysql_fix_privilege_tables.sql file and use that to upgrade
the system tables of MySQL
- Check for any unexpected error returned from runnning the mysql_fix_privilege_tables SQL
- Fix bug#26639, bug#24248 and bug#25405
