fixed test func_gconcat to be repeatable independent of presend tables and avoid removing user tables

(SCRUM)

initialization of item_thd moved to constructor (in any case we need thd in constructor)
initialization of group_concat_max_len to constructor to avoid incorrect length reporting (BUG#757)
removed Item_func_group_concat::fix_length_and_dec() because item have its own fix_fields and will not have inherited items

into myvenu.com:/home/venu/work/sql/dev-4.1

into bar.mysql.r18.ru:/usr/home/bar/mysql-4.1

  Item_func::agg_arg_collations()
  Item_func::arr_arg_collations_for_comparison()
 to aggregate argument collations.
It helps to reuse a lot of code.

into serg.mylan:/usr/home/serg/Abk/mysql-4.1

into mysql.r18.ru:/usr/home/ram/work/4.1

into serg.mylan:/usr/home/serg/Abk/mysql-4.1

into narttu.mysql.fi:/my/mysql-4.1

into bar.mysql.r18.ru:/usr/home/bar/mysql-4.1

into sanja.is.com.ua:/home/bell/mysql/bk/work-4.1

  ROW('a','b','c) = ROW('A' collate latin1_bin,'b','c') returns 0
When a number is compared to a string, character sets and 
collations are not aggregated. e.g. this returned error in 4.1.0:

SELECT 1=_latin2'1';
because character sets was aggregated, and 1 was considered as
a string of latin1 charset during this aggregation.

into narttu.mysql.fi:/my/mysql-4.1

into sanja.is.com.ua:/home/bell/mysql/bk/work-4.1

It looks like samll revolution in SELECT_LEX tree, but it was only natural way to solve problem with name resolution of external fields inside subselect which belongs to global order of union
also it have following advantages:
 - removed mess with current_select type conversion
   - type checking/converting
   - a lot of virtual methods
 - fake select for union execution allocated only once (it was allocated for every subselect with union executing)

changes:
fixed bug with outer fields name resolution of subqueries which belong to global ORDER BY clause
remuved select_lex() function, now thd->lex.current_select always have type SELECT_LEX
new SELECT_LEX (fake_select_lex) will be allocated in case of UNION for using in UNION processing
fake_select_lex allocated for union hold global ORDER BY & LIMIT clauses and have linkage equal to GLOBAL_OPTIONS_TYPE 
new description of SELECT_LEX tree

(SCRUM)

fixed dependence of items from reduced subquery
(SCRUM)

removed outer resolving flag (because of movingtransformation after fix_fields)

into sanja.is.com.ua:/home/bell/mysql/bk/work-err-4.1

into mashka.mysql.fi:/home/my/mysql-4.1