It's not possible to flush the global status variables in 5.0
Update test case so it works by recording the value of handle_rollback
before and compare it to the value after


mysql-test/r/innodb_mysql.result:
  Update result file
mysql-test/t/innodb_mysql.test:
  It's not possible to reset the global status variables in 5.0 so intead its value
  is recorded and compared to the after value.
  It should not have changed.
sql/mysqld.cc:
  Improve comments
sql/set_var.cc:
  Improve comments
sql/sql_class.cc:
  Improve comments
sql/sql_class.h:
  Improve comments