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

tests/mysql_client_test.c

@@ -11801,8 +11801,8 @@ static void test_bug6046()
 
   stmt= mysql_stmt_init(mysql);
 
-  stmt_text= "SELECT t1.a FROM t1 NATURAL JOIN t1 as X1 "
-             "WHERE t1.b > ? ORDER BY t1.a";
+  stmt_text= "SELECT a FROM t1 NATURAL JOIN t1 as X1 "
+             "WHERE b > ? ORDER BY a";
 
   rc= mysql_stmt_prepare(stmt, stmt_text, strlen(stmt_text));
   check_execute(stmt, rc);