Added variable "OLD_MODE" that can be used to turn off the new behavior

mysql-test/r/insert.result:
  Added test case
mysql-test/r/mysqld--help.result:
  Added old_mode
mysql-test/suite/sys_vars/r/old_mode_basic.result:
  Added testing of new variable
mysql-test/suite/sys_vars/t/old_mode_basic.test:
  Added testing of new variable
mysql-test/t/insert.test:
  Added test case
sql/sql_class.h:
  Added bit flags for OLD_MODE
sql/sql_insert.cc:
  Disable duplicate key warnings for INSERT IGNORE of OLD_MODE NO_DUP_KEY_WARNINGS_WITH_IGNORE is used
sql/sql_show.cc:
  Don't show progress reporting on SHOW PROCESSLIST if OLD_MODE NO_PROGRESS_INFO is used
sql/sys_vars.cc:
  Added OLD_MODE