Faster alter table code for 5.1.

mysql-test/r/alter_table.result:
  Added some additional tests for new alter table code.
mysql-test/t/alter_table.test:
  Added some additional tests for new alter table code.
sql/field.cc:
  Functions to check whether new field is equal with old field.
  Classes for different types.
sql/field.h:
  Functions to check whether new field is equal with old field.
  Classes for different types.
sql/ha_berkeley.cc:
  check_if_incompatible_data() for BDB.
sql/ha_berkeley.h:
  check_if_incompatible_data() for BDB.
sql/ha_heap.cc:
  check_if_incompatible_data() for HEAP.
sql/ha_heap.h:
  check_if_incompatible_data() for HEAP.
sql/ha_innodb.cc:
  check_if_incompatible_data() for InnoDB.
sql/ha_innodb.h:
  check_if_incompatible_data() for InnoBD.
sql/ha_myisam.cc:
  check_if_incompatible_data() for MyISAM.
sql/ha_myisam.h:
  check_if_incompatible_data() for MyISAM.
sql/ha_myisammrg.cc:
  check_if_incompatible_data() for Merge tables.
sql/ha_myisammrg.h:
  check_if_incompatible_data() for Merge tables.
sql/ha_ndbcluster.cc:
  check_if_incompatible_data() for NDB.
sql/ha_ndbcluster.h:
  check_if_incompatible_data() for NDB.
sql/handler.h:
  Defines for COMPATIBLE_DATA (yes and no) and the default function
  for check_if_incompatible_data().
sql/mysql_priv.h:
  Defines for IS_EQUAL_*
sql/mysqld.cc:
  Added option --old-alter-table to disable new alter table code.
sql/set_var.cc:
  Added option --old-alter-table to disable new alter table code.
sql/set_var.h:
  Added option --old-alter-table to disable new alter table code.
sql/sql_class.h:
  Added option --old-alter-table to disable new alter table code.
sql/sql_lex.h:
  Added a flag for forcing recreation of a table
  (needed for optimize table mapped to alter table)
sql/sql_table.cc:
  Made a function of setting table default charset, used
  now in two places.
  
  Added defines for ALTER_TABLE_* possible changes.
  Currently just overall data and index.
  
  Added function compare_tables, which checks fields
  compatibility in old and new tables.
BitKeeper/etc/config:
  Disabled logging

sql/sql_class.h

@@ -564,6 +564,7 @@ struct system_variables
   my_bool ndb_use_exact_count;
   my_bool ndb_use_transactions;
 #endif /* HAVE_NDBCLUSTER_DB */
+  my_bool old_alter_table;
   my_bool old_passwords;
   
   /* Only charset part of these variables is sensible */