- Make the code produce correct result: use an array of triggers to turn on/off equalities for each
  compared column. Also turn on/off optimizations based on those equalities.
- Make EXPLAIN output show "Full scan on NULL key" for tables for which we switch between
  ref/unique_subquery/index_subquery and ALL access.
- index_subquery engine now has HAVING clause when it is needed, and it is
  displayed in EXPLAIN EXTENDED
- Fix incorrect presense of "Using index" for index/unique-based subqueries (BUG#22930)
// bk trigger note: this commit refers to BUG#24127


mysql-test/r/ndb_subquery.result:
  BUG#24127: wrong result for (null,not-null) IN (SELECT a,b ...)
  - Updated test results (checked)
mysql-test/r/subselect.result:
  BUG#24127: wrong result for (null,not-null) IN (SELECT a,b ...)
  - Updated test results (checked)
mysql-test/r/subselect2.result:
  BUG#24127: wrong result for (null,not-null) IN (SELECT a,b ...)
  - Updated test results (checked)
mysql-test/r/subselect3.result:
  BUG#24127: wrong result for (null,not-null) IN (SELECT a,b ...)
  - Testcases
mysql-test/t/subselect3.test:
  BUG#24127: wrong result for (null,not-null) IN (SELECT a,b ...)
  - Testcases
sql/item_cmpfunc.cc:
  BUG#24127: wrong result for (null,not-null) IN (SELECT a,b ...)
  - For row-based IN subqueries, use one flag per each column. Set the flags appropriately before
    running the subquery.
sql/item_cmpfunc.h:
  BUG#24127: wrong result for (null,not-null) IN (SELECT a,b ...)
   - Added Item_func_trig_cond::get_triv_var()
sql/item_subselect.cc:
  BUG#24127: wrong result for (null,not-null) IN (SELECT a,b ...)
  - Item_subselect::exec() and subselect_*_engine::exec() don't have parameter
    anymore - now Item_subselect owns the pushed down predicates guard flags.
  - A correct set of conditional predicates is now pushed into row-based IN 
    subquery.
  - select_indexsubquery_engine now has "HAVING clause" (needed for correct query
    results), and it is shown in EXPLAIN EXTENDED
sql/item_subselect.h:
  BUG#24127: wrong result for (null,not-null) IN (SELECT a,b ...)
  - Item_subselect::exec() and subselect_*_engine::exec() don't have parameter
    anymore - now Item_subselect owns the pushed down predicates guard flags.
  - A correct set of conditional predicates is now pushed into row-based IN 
    subquery.
  - select_indexsubquery_engine now has "HAVING clause" (needed for correct query
    results), and it is shown in EXPLAIN EXTENDED
sql/mysql_priv.h:
  BUG#24127: wrong result for (null,not-null) IN (SELECT a,b ...)
  - Added "in_having_cond" special Item name
sql/mysqld.cc:
  BUG#24127: wrong result for (null,not-null) IN (SELECT a,b ...)
  - Added "in_having_cond" special Item name
sql/sql_lex.h:
  BUG#24127: wrong result for (null,not-null) IN (SELECT a,b ...)
sql/sql_select.cc:
  BUG#24127: wrong result for (null,not-null) IN (SELECT a,b ...)
  - Make "ref" analyzer be able to work with conditional equalities
  - Fix subquery optimization code to match the changes in what kinds of 
    conditions are pushed down into subqueries 
  - Fix wrong EXPLAIN output in some queries with subquery (BUG#22390)
sql/sql_select.h:
  BUG#24127: wrong result for (null,not-null) IN (SELECT a,b ...)
  - Make "ref" analyzer be able to work with conditional equalities
  - Fix wrong EXPLAIN output in some queries with subquery (BUG#22390)