returns too many rows

Patch #2
 - address Guilhem's comments
 - added optimizer trace changes supplied by Guilhem.

mysql-test/r/subquery_sj_all_bka_nixbnl.result
mysql-test/r/subquery_sj_mat.result
mysql-test/r/subquery_sj_mat_bka.result
mysql-test/r/subquery_sj_mat_bka_nixbnl.result
mysql-test/r/subquery_sj_mat_bkaunique.result
  EXPLAIN changes due to no longer checking that a keyuse->val->type()
  is Item::FIELD_ITEM within JOIN::update_equalities_for_sjm().

mysql-test/suite/opt_trace/include/general.inc
  Modified optimizer trace settings for preservation of existing
  behaviour.

mysql-test/suite/opt_trace/r/bugs_no_prot_all.result
mysql-test/suite/opt_trace/r/bugs_ps_prot_all.result
mysql-test/suite/opt_trace/r/general_no_prot_all.result
mysql-test/suite/opt_trace/r/general_no_prot_none.result
mysql-test/suite/opt_trace/r/general_ps_prot_all.result
mysql-test/suite/opt_trace/r/general_ps_prot_none.result
  Changed optimizer trace results.

sql/item_cmpfunc.cc
  Code simplifications and added comments.
  Check on whether an equality originates from a semi-join equality
  is slightly changed.

sql/item_subselect.cc
  Replaced use of TABLE_LIST::materialized with JOIN_TAB::materialized.

sql/opt_explain.cc
  Semijoin_mat_exec pointer is taken from JOIN_TAB, not TABLE_LIST.
  Use inner_table_index instead of old table_index.

sql/opt_range.cc
  Reverted use of JOIN::plan_is_single_table().

sql/sql_class.h
  In class Semijoin_mat_exec, added field mat_table_index, and renamed
  table_index to inner_table_index. inner_table_index is also made an
  absolute index (it used to be relative to mat_table_index).
  Member field_count is deleted.
  subq_exprs is converted from being a List to pointer to List.

sql/sql_executor.cc
  In end_sj_materialize(), Semijoin_mat_exec is taken from JOIN_TAB.
  Some comment changes.
  join_materialize_table() is renamed to join_materialize_derived().
  join_materialize_semijoin() zeroes last->next_select and
  last->sj_mat_exec after use.
  inner_table_index is converted to an absolute index.

sql/sql_executor.h
  join_materialize_table() is renamed to join_materialize_derived().

sql/sql_join_buffer.cc
  Comment added inside JOIN_CACHE::calc_record_fields().

sql/sql_optimizer.cc
  Logic of eliminate_item_equal() is rewritten according to suggestion
  from Guilhem.
  JOIN::update_keyuse() is incorporated into JOIN::update_equalities(),
  which is renamed to JOIN::update_equalities_for_sjm().
  Semijoin_mat_exec pointer is taken from JOIN_TAB instead of TABLE_LIST.
  JOIN::allocate_sj_mat_exec() is incorporated into
  JOIN::get_best_combination().
  Local struct KEY_FIELD is renamed to Key_field in association with
  introduction of constructor.
  create_keyuse_for_table() now uses add_key_part() when creating keyuse
  array.

sql/sql_optimizer.h
  Removed member sjm_nests from class JOIN, it is calculated locally.
  update_equalities() is renamed to update_equalities_for_sjm().
  Updated comments.
  
sql/sql_select.cc
  In clear_sj_tmp_tables(), added back clearing of materialized flag for
  semi-joined materialized tables.
  JOIN::destroy() now deletes all Semijoin_mat_exec objects.
  JOIN::get_best_combination(): Logic slightly re-arranged, creates the
  list of Semijoin_mat_exec objects.
  In JOIN::setup_materialized_table(), take Semijoin_mat_exec from
  JOIN_TAB, increment name allocation by 1 byte for null termination,
  use TABLE_LIST::init_one_table() to initialize object, modify "fanout"
  calculation for materialized temporary table, use
  POSITION::set_prefix_costs() to set cost.
  JOIN_TAB::cleanup(), earlier valgrind compensation deleted.

sql/sql_select.h
  Added Semijoin_mat_exec pointer to class JOIN_TAB.

sql/sql_show.cc
  Cosmetic changes.