Fixed query plans with loose index scan degraded into index scan.

Analysis:
With MWL#89 subqueries are no longer executed and substituted during
the optimization of the outer query. As a result subquery predicates
that were previously executed and substituted by a constant before
the range optimizer were present as regular subquery predicates during
range optimization. The procedure check_group_min_max_predicates()
had a naive test that ruled out all queries with subqueries in the
WHERE clause. This resulted in worse plans with MWL#89.

Solution:
The solution is to refine the test in check_group_min_max_predicates()
to check if each MIN/MAX argument is referred to by a subquery predicate.

Analysis:
Close to its end JOIN::optimize() assigns having to tmp_having, and
sets the having clause to NULL:

  tmp_having= having;
  if (select_options & SELECT_DESCRIBE)
  {
    error= 0;
    DBUG_RETURN(0);
  }
  having= 0;

At the same time, this query detects an empty result set, and calls
return_zero_rows(), which checks the HAVING clause as follows:

    if (having && having->val_int() == 0)
      send_row=0;

However having has been already set to NULL, so return_zero_rows
doesn't check the having clause, hence the wrong result.

Solution:
Check join->tmp_having in addition to join->having.

There is no additional test case, because the failure was in
the current regression test.

This fixes the wrong result in tests like compress, join, join_cache, greedy_optimizer and select_pkeycache

When stored in a key buffer any varchar field has a length prefix
that always takes 2 bytes.

Added a test case for bug #695442 - a duplicate of bug 694092.

The bug was the result of a bad merge maria-5.2-wl21 -> 5.3.

One of the hash functions employed by the BNLH join algorithm
calculates the the value of hash index for key value utilizing
every byte of the key buffer. To make this calculation valid
one has to ensure that for any key value unused bytes of the 
buffer are filled with with a certain filler. We choose 0 as
a filler for these bytes.

Added an optional boolean parameter with_zerofill to the function
key_copy. If the value of the parameter is TRUE all unused bytes
of the key buffer is filled with 0. 

In some cases the function make_cond_for_index() was mistaken
when detecting index only pushdown conditions for a table: 
a pushdown condition that was not index only could be marked
as such.
It happened because the procedure erroneously used the markers
for index only conditions that remained from the calls of
this function that extracted the index conditions for other 
tables.
Fixed by erasing index only markers as soon as they are need
anymore.

  
- Post-review fixes. Intermediate commit to address review point 1.6.
- Fixed valgrind warnings

Lifted the limitation that hash join could not be used over 
varchar fields with non-binary collation.

- Merge with 5.3-main

set thd->password appropriately also for cases when a user was not found.

- Fixes for the second fix: take into account case where we don't need to save/restore the scan.

Switch from "Disable identical key handling optimization when
  IndexConditionPushdown is used" approach
To 
 an approach where we save/restore index tuple and so can use index condition pushdown.

mwl#21.

It was missing from EXTRA_DIST in Makefile.am.

 - Address Monty's review feedback, portion 3

- Address Monty's review feedback, portion 2

Post-review fixes. Intermediate commit to address
review points 1.1, 1.2, 1.3, 1.4, 1.5, and 3.1, 3.2, 3.3.

- Better warnings

Analysis:
The assert failed because st_select_lex::print() was called for subqueries
as follows:

Item_subselect::print() ->
  subselect_single_select_engine::print() -> st_select_lex::print()

It was Item_subselect::fix_fields() that set the thd by calling set_thd(),
so when this print() was called before fix_fields(), subselect_engine::thd
was NULL.

Solution:
The patch makes all constructors of all subselect_engine classes to take
a THD parameter. The default subselect_single_select_engine engine is created
early during parse time, in the Item_subselect::init call, so we pass the
correct THD object already at this point.

- Address review feedback: change return type of RANGE_SEQ_IF::next()

(Includes patch for overrun detected by valgrind thanks to previous my_alloca() -> my_malloc() patch)

mysys/thr_lock.c:
  Fixed typo that caused compile failure

- This allows us to detect missing my_afree() calls and also find overruns (when running with valgrind) to alloca() areas.
- Added missing my_afree() calls
- Fixed wrong call to my_afree()


include/my_sys.h:
  When compiling with valgrind, change my_alloca() to use my_malloc()
mysql-test/suite/innodb/t/innodb_bug57255.test:
  Speed up taste case (patch from Stewart Smith)
mysql-test/suite/innodb_plugin/t/innodb_bug57255.test:
  Speed up taste case (patch from Stewart Smith)
sql/ha_partition.cc:
  Removed casts from my_afree()
sql/opt_range.cc:
  Add missing my_afree() calls.
storage/maria/ma_rt_split.c:
  Fixed wrong parameter to my_afree()