added "nulls_ignored" index statistics collection method for MyISAM tables.
(notification trigger: this is about BUG#9622).


include/my_base.h:
  BUG#9622: Added MI_STATS_METHOD_IGNORE_NULLS statistics collection method:
  Added SEARCH_RETURN_B_POS flag for ha_key_cmp()
include/my_handler.h:
  BUG#9622: Added MI_STATS_METHOD_IGNORE_NULLS statistics collection method: added ha_find_null()
include/myisam.h:
  BUG#9622: Added MI_STATS_METHOD_IGNORE_NULLS statistics collection method.
myisam/mi_check.c:
  BUG#9622: Added MI_STATS_METHOD_IGNORE_NULLS statistics collection method, added 
   mi_collect_stats_*(), updated update_key_parts() to deal with all 3 methods.
myisam/myisamchk.c:
  BUG#9622: Added nulls_ignored index statistics collection method for MyISAM
myisam/myisamdef.h:
  BUG#9622: Added MI_STATS_METHOD_IGNORE_NULLS statistics collection method.
myisam/sort.c:
  BUG#9622: Added MI_STATS_METHOD_IGNORE_NULLS statistics collection method.
mysql-test/r/myisam.result:
  Testcase for BUG9622
mysql-test/t/myisam.test:
  Testcase for BUG9622
mysys/my_handler.c:
  BUG#9622: ha_key_cmp() now supports new SEARCH_RETURN_B_POS flag, added ha_find_null()
sql/ha_myisam.cc:
  BUG#9622: Added MI_STATS_METHOD_IGNORE_NULLS statistics collection method.
sql/mysqld.cc:
  BUG#9622: Added MI_STATS_METHOD_IGNORE_NULLS statistics collection method.