Task ID 894:
Backport default_week_format variable to 4.0


mysql-test/r/func_time.result:
  Backport default_week_format variable to 4.0
mysql-test/t/func_time.test:
  Backport default_week_format variable to 4.0
sql/mysqld.cc:
  Backport default_week_format variable to 4.0
sql/set_var.cc:
  Backport default_week_format variable to 4.0
sql/sql_class.h:
  Backport default_week_format variable to 4.0
sql/sql_yacc.yy:
  Backport default_week_format variable to 4.0