This is mostly needed to hide all references to free_tables, so that further
implementation of multi-instance list can be done completely inside
table_cache.cc

Allowing GROUP_CONCAT(... ORDER BY ..) in queries with ROLLUP.

The story of the related code:

1. The original patch from Wax
commit: 0b505fb4
date: Tue Mar 18 03:07:40 2003

opt_gorder_clause reused the regular order_clause,
which already had some protection against ROLLUP queries:

order_clause:
        ORDER_SYM BY
        {
          LEX *lex=Lex;
          if (lex->current_select->linkage != GLOBAL_OPTIONS_TYPE &&
              lex->current_select->select_lex()->olap !=
              UNSPECIFIED_OLAP_TYPE)
          {
            net_printf(lex->thd, ER_WRONG_USAGE,
                       "CUBE/ROLLUP",
                       "ORDER BY");
            YYABORT;
          }
        } order_list;

The assumption that ORDER BY in group_concat() had to have
the same ROLLUP restriction (with order_clause) was wrong.

Moreover, GROUP_CONCAT() in select_item_list was not affected
by this restriction, because WITH ROLLUP goes after
select_item_list and therefore sel->olap is always equal
to UNSPECIFIED_OLAP_TYPE during select_item_list.

GROUP BY was not affected for two reasons:
- it goes before WITH ROLLUP and sel->olap is still
  UNSPECIFIED_OLAP_TYPE
- Aggregate functions like AVG(), GROUP_CONCAT() in GROUP BY
  are not allowed

So only GROUP_CONCAT() in HAVING and ORDER BY clauses
were erroneously affected by this restriction.

2. Bug#27848 rollup in union part causes error with order of union
commit: 3f6073ae
Author: unknown <igor@olga.mysql.com>  2007-12-15 01:42:46

The condition in the ROLLUP protection code became more complex.
Note, opt_gconcat_order still reused the regular order_clause.

3. Bug#16347426 ASSERTION FAILED: (SELECT_INSERT &&
                 !TABLES->NEXT_NAME_RESOLUTION_TABLE) || !TAB
commit: 2d836633
author: Chaithra Gopalareddy <chaithra.gopalareddy@oracle.com>
date: 2013-04-14 06:00:49

opt_gorder_clause was refactored not to use order_clause and
to collect information directly to select->gorder_list.
The ROLLUP protection code was duplicated from order_clause
to the new version of opt_gorder_clause.

- Validate the specified wsrep_start_position value by also
checking the return status of wsrep->sst_received. This also
ensures that changes in wsrep_start_position is not allowed
when the node is not in JOINING state.
- Do not allow decrease in seqno within same UUID.
- The initial checkpoint in SEs should be [0...:-1].

Using /*empty*/, union_list, union_order_or_limit instead.
This is to get rid of lex->current_select->braces easier
(separately in union_list and in union_order_or_limit)

UNION, ROLLUP, GROUP_CONCAT, for better coverage.

MDEV-10102 Disallow CREATE VIEW .. PROCEDURE ANALYSE() syntactically

- Moving opt_union_order_or_limit inside union_opt,
  as it's not used in other places any more.
- Changing union_opt to have no type.
  Earlier (before all MDEV-8909 dependency tasks) it had the <num> type,
  and it's return value was used to generate errors.
  Now union_opt does not need a return value because the grammar
  disallows ORDER and LIMIT clauses in wrong context.

MDEV-10095 Fix derived tables to return a syntax error instead of "Illegal usage of UNION and LIMIT"

from select_derived_init.

(to simplify further MDEV-8909 changes)

1. Better semantic readability:
- Moving get_select_lex_derived inside select_derived_init
  and decomposing it into get_select_lex and
  $1->init_nested_join(lex->thd)
-  Moving DBUG_ASSERT($1 == Lex->current_select) inside
   select_derived_init

Now init_nested_join() and end_nested_join() reside inside
the same rule select_derived_init.
(It's very likely that they can be further removed,
 as there are no any joins in this rule).

3. Better grammar readability:
  Moving SELECT_SYM from select_derived_init to derived_query_specification.
  It's easier to read a rule when it starts from a terminal symbol.

1. Moving the "| get_select_lex_derived select_derived_init" part of
select_derived into a separate new rule derived_query_specification.
2. Using derived_query_specification directly in select_derived_union
rather than in select_derived.
3. Moving the sequence "opt_order_clause opt_limit_clause opt_select_lock_type"
from select_derived2 to select_derived_union,
after derived_query_specification.

Effectively, the parser now does not go through the sequence
"opt_order_clause opt_limit_clause ... opt_union_order_or_limit" any more.

This fixes the problem with double LIMIT clause and removes 2 shift/reduce
conflicts.

"MDEV-8909 union parser cleanup" changes.

When the server parses a query like
  SELECT * FROM (SELECT 1);
a sequence of calls
  st_select_lex::init_nested_join() ..
  st_select_lex::end_nested_join()
was performed two times (tested in gdb).
Both pairs of calls seem to be redundant for a query
like this, because there are actually no any joins here.

This patch moved "table_ref_select" inside "select_derived",
which revealed that one the pairs was definitely redundant:
After this transformation we got an init_nested_join()
immediately followed by end_nested_join() for the same st_select_lex,
which has no any sense. So this pair of calls was removed.

to simplify further MDEV-8909 refactoring.

This was a bug in the parser. As a result it could accept queries
with invalid derived tables if they used With clauses.

- Moving "SELECT_SYM select_init2_derived opt_table_expression"
  from query_term into a new separate rule query_specification,
  and using query_specification in the beginning of query_term.
- query_term now does not have a %type, query_specification has a %type
  instead. This removes duplicate code that returns
  Lex->current_select->master_unit()->first_select();

between select_derived_union and query_expression_body.
An upcoming patch for MDEV-10035 will also reuse union_head_non_top.

- Moving select_options_and_item_list from select_init2
  to select_init and view_select_aux
- Renaming select_init2 to select_init3
This will simplify upcoming sql_yacc.yy fixes (e.g. MDEV-10035, MDEV-8909).

An addition to original patch:
- use FN_REFLEN instead of HOST_NAME_MAX (the latter can be undefined)
- avoid calling gethostname() on every prompt construction by caching it

Introduce `\H` option which behaves mostly like `\h`. The only exception is
when the client connects to the server hosted on localhost. In this case, the
hostname will be used instead.

closer the grammar in the SQL Standard:
- <query specification> is only a SELECT followed by
  <set quantifier>, <select list> and <table expression>.
- While <query term> includes SELECT queries and derived tables.

MDEV-10036 sql_yacc.yy: Split select_part2 to disallow syntactically bad constructs with INTO, PROCEDURE, UNION
MDEV-10037 UNION with LIMIT ROWS EXAMINED does not require parentheses

MDEV-10030 sql_yacc.yy: Split table_expression and remove PROCEDURE from create_select, select_paren_derived, select_derived2, query_specification

This change refactors the "table_expression" rule in sql_yacc.yy.

Queries with subselects and derived tables, as well as "CREATE TABLE ... SELECT"
now return syntax error instead of "Incorrect usage of PROCEDURE and ...".