Implementation of WL#2486 -

"Process NATURAL and USING joins according to SQL:2003".

* Some of the main problems fixed by the patch:
  - in "select *" queries the * expanded correctly according to
    ANSI for arbitrary natural/using joins
  - natural/using joins are correctly transformed into JOIN ... ON
    for any number/nesting of the joins.
  - column references are correctly resolved against natural joins
    of any nesting and combined with arbitrary other joins.

* This patch also contains a fix for name resolution of items
  inside the ON condition of JOIN ... ON - in this case items must
  be resolved only against the JOIN operands. To support such
  'local' name resolution, the patch introduces a stack of
  name resolution contexts used at parse time.

NOTICE:
- This patch is not complete in the sense that
  - there are 2 test cases that still do not pass -
    one in join.test, one in select.test. Both are marked
    with a comment "TODO: WL#2486".
  - it does not include a new test specific for the task

sql/sql_class.h

@@ -1217,6 +1217,12 @@ public:
   ulonglong  limit_found_rows;
   ha_rows    cuted_fields,
              sent_row_count, examined_row_count;
+  /*
+    The set of those tables whose fields are referenced in all subqueries
+    of the query.
+    TODO: possibly this it is incorrect to have used tables in THD because
+    with more than one subquery, it is not clear what does the field mean.
+  */
   table_map  used_tables;
   USER_CONN *user_connect;
   CHARSET_INFO *db_charset;