- 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