Preparatory (and the most problematic) patch for Bug#7306

"the server side preparedStatement error for LIMIT placeholder",
which moves all uses of LIMIT clause from PREPARE to OPTIMIZE
and later steps.
After-review fixes.


mysql-test/r/group_min_max.result:
  Test results fixed for EXPLAINs when using GROUP_MIN_MAX access plan.
sql/item_subselect.cc:
  Move setting of the internal LIMIT used for IN/ALL/ANY/EXISTS 
  subqueries to one place: Item_exists_subselect::fix_length_and_dec().
  This implies that unit->select_limit_cnt is not set until the item is 
  fixed. This is OK, as now LIMIT values are not used until JOIN::optimize.
sql/mysql_priv.h:
  setup_tables no longer needs a special flag for the case when
  it's called from JOIN::reinit() (we don't need to call setup_tables
  between two executions of a correlated subquery).
sql/opt_range.cc:
  Fix a glitch in GROUP_MIN_MAX access plan: we should use table metadata,
  not field data, to evaluate max_used_key_length, which is then
  used for explain.
sql/sp.cc:
  - setup_tables signature changed.
sql/sql_base.cc:
  - setup_tables no longer needs a special mode for subqueries.
    Unused checks were removed.
sql/sql_delete.cc:
  - setup_tables signature changed
sql/sql_help.cc:
  - setup_tables signature changed
sql/sql_insert.cc:
  - setup_tables signature changed
sql/sql_lex.cc:
  Consolidate setting of internal LIMIT for IN/ALL/ANY/EXISTS subqeries
  in one place, and hence remove it from st_select_lex::test_limit().
sql/sql_lex.h:
  Cleanup signature of st_select_lex_unit::init_prepare_fake_select_lex().
sql/sql_load.cc:
  - setup_tables signature changed
sql/sql_olap.cc:
  - setup_tables signature changed
sql/sql_parse.cc:
  - st_select_lex_unit::set_limit() signature changed
sql/sql_select.cc:
  Move setting of JOIN::select_limit from JOIN::prepare
  to JOIN::optimize. At prepare, limit is unknown yet.
  Remove excessive cleanups from JOIN::reinit which were overwriting
  join->join_tab[i]->table->used_keys. This fixes the bug which was triggered
  by the change in item_subselect.cc.
sql/sql_union.cc:
  Class st_select_lex_unit was changed to avoid calls to 
   st_select_lex_unit::set_limit from places where it may be unknown.
  Now unit->select_limit_cnt is set at ::exec(). 
  st_select_lex_unit::init_prepare_fake_select_lex(): 
   - move out set_limit functionality
   - remove a few lines of dead code.
  st_select_lex_unit::prepare():
    - now we don't call set_limit at the time of prepare, so the value  
      of unit->select_limit_cnt may be unknown here. Use sl->select_limit
      instead.
  st_select_lex_unit::exec():
    - cleanup
    - call set_limit explicitly as it has been moved out of 
     init_prepare_fake_select_lex.
sql/sql_update.cc:
  - setup_tables signature changed

sql/sql_help.cc

@@ -651,7 +651,7 @@ bool mysqld_help(THD *thd, const char *mask)
 
     tables do not contain VIEWs => we can pass 0 as conds
   */
-  setup_tables(thd, tables, 0, &leaves, FALSE, FALSE);
+  setup_tables(thd, tables, 0, &leaves, FALSE);
   memcpy((char*) used_fields, (char*) init_used_fields, sizeof(used_fields));
   if (init_fields(thd, tables, used_fields, array_elements(used_fields)))
     goto error;