renamed "--old-mode" to "--old" to prevent
 ambiguity.
 "old" now appears in SHOW VARIABLES as a
 read-only option.


mysql-test/r/group_by.result:
  WL3527: 5.1 test case
mysql-test/t/group_by.test:
  WL3527: 5.1 test case
sql/mysqld.cc:
  WL3527: 5.1
   renamed the "old-mode" to "old" to fit
   the options naming scheme
sql/set_var.cc:
  WL3527: 5.1
   added "-old" as read-only system variable
sql/set_var.h:
  WL3527: 5.1
   added class for boolean pointer read-only 
   option to support the "--old" option.
sql/sql_base.cc:
  fixed 5.0->5.1 merge problems.