The problem is that when upgrading the event table, the sql_mode
column was always being added instead of being updated to list new
sql_mode values, causing upgrades of the event which already have
a sql_mode column to not be updated to the new sql_mode values.

The solution is to always add first a stub sql_mode column and
subsequently update the column to the new sql_mode values.


scripts/mysql_system_tables_fix.sql:
  Always add and update the sql_mode column of the event table.