statistics (like 4.0 did) (patch #3, with review #1 & #2 feedback addressed)


include/myisam.h:
  BUG#12232: Add a server option to treat NULL values as equal when calculating MyISAM index statistics
myisam/mi_check.c:
  BUG#12232: Add a server option to treat NULL values as equal when calculating MyISAM index statistics
myisam/myisamchk.c:
  BUG#12232: Add a server option to treat NULL values as equal when calculating MyISAM index statistics
mysql-test/r/myisam.result:
  BUG#12232: Add a server option to treat NULL values as equal when calculating MyISAM index statistics
mysql-test/t/myisam.test:
  BUG#12232: Add a server option to treat NULL values as equal when calculating MyISAM index statistics
sql/ha_myisam.cc:
  BUG#12232: Add a server option to treat NULL values as equal when calculating MyISAM index statistics
sql/handler.h:
  BUG#12232: Add a server option to treat NULL values as equal when calculating MyISAM index statistics
sql/mysqld.cc:
  BUG#12232: Add a server option to treat NULL values as equal when calculating MyISAM index statistics
sql/set_var.cc:
  BUG#12232: Add a server option to treat NULL values as equal when calculating MyISAM index statistics
sql/sql_class.h:
  BUG#12232: Add a server option to treat NULL values as equal when calculating MyISAM index statistics