Warnings elimination

There were two problems when inferring the correct field types resulting from
UNION queries.
- If the type is NULL for all corresponding fields in the UNION, the resulting 
  type would be NULL, while the type is BINARY(0) if there is just a single 
  SELECT NULL.
- If one SELECT in the UNION uses a subselect, a temporary table is created
  to represent the subselect, and the result type defaults to a STRING type,
  hiding the fact that the type was unknown(just a NULL value).
Fixed by remembering whenever a field was created from a NULL value and pass
type NULL to the type coercion if that is the case, and creating a string field
as result of UNION only if the type would otherwise be NULL.

updated result files

into  mysql.com:/home/gluh/MySQL/Merge/5.0-opt

issue an error if string has illegal characters

into  gleb.loc:/home/uchum/work/bk/5.0-opt

  Fixed dbug macros usage errors

into  magare.gmz:/home/kgeorge/mysql/autopush/B31156-5.0-opt

into  magare.gmz:/home/kgeorge/mysql/autopush/B31440-5.0-opt

into  gleb.loc:/home/uchum/work/bk/5.0-opt

into  magare.gmz:/home/kgeorge/mysql/autopush/B31440-5.0-opt

into  mysql.com:/home/hf/work/31155/my50-31155

into  gleb.loc:/home/uchum/work/bk/5.0-opt

Bug#30982 CHAR(..USING..) can return a not-well-formed string
Bug#30986 Character set introducer followed by a HEX string can return bad result
check_well_formed_result moved to Item from Item_str_func
fixed Item_func_char::val_str for proper ucs symbols converting
added check for well formed strings for correct conversion of constants with underscore
charset

The special case with NULL as a regular expression
was handled at prepare time. But in this special case
the item was not marked as fixed. This caused an assertion
at execution time.
Fixed my marking the item as fixed even when known to 
return NULL at prepare time.

                  precision > 0 && scale <= precision'.

A sign of a resulting item of the IFNULL function was not
updated and the maximal length of this result was calculated
improperly. Correct algorithm was copy&pasted from the IF
function implementation.

into  mysql.com:/home/gluh/MySQL/Merge/5.0-opt

added variables relay_log, relay_log_index, relay_log_info_file to init_vars[]
to make them visible within SHOW VARIABLES

removed now() call to make the test to be year independent

into  dl145s.mysql.com:/data0/mhansson/bug31160/my50-bug31160

into  dl145s.mysql.com:/data0/mhansson/bug31160/my50-bug31160

Even though it returns NULL, the MAKETIME function did not have this property set,
causing a failed assertion (designed to catch exactly this).
Fixed by setting the nullability property of MAKETIME().

into  linux-st28.site:/home/martin/mysql/src/bug30832-again-push/my50-bug30832-again

Completion of previous patch. Negative number were denied
as the second argument to NAME_CONST.

  Item_sum_distinct::setup(THD*): Assertion

There was an assertion to detect a bug in ROLLUP
implementation. However the assertion is not true
when used in a subquery context with non-cacheable
statements.
Fixed by turning the assertion to accepted case
(just like it's done for the other aggregate functions). 

when a divisor is less than 1 and its fractional part is very long.
For example:
1 % .123456789123456789123456789123456789123456789123456789123456789123456789123456789;

Stack buffer overflow has been fixed in the do_div_mod function.

into  mysql.com:/home/hf/work/30286/my50-30286

into  dl145s.mysql.com:/data0/mhansson/bug30942/my50-bug30942

into  mysql.com:/home/hf/work/30286/my50-30286

into  mysql.com:/home/hf/work/30286/my41-30286

into  mysql.com:/home/hf/work/30286/my50-30286

into  mysql.com:/home/hf/work/30286/my41-30286

into  mysql.com:/home/hf/work/30286/my50-30286

into  mysql.com:/home/hf/work/30286/my50-30286

As the result of DOUBLE claculations can be bigger
than DBL_MAX constant we use in code, we shouldn't use this constatn
as a biggest possible value.
Particularly the rtree_pick_key function set 'min_area= DBL_MAX' relying
that any rtree_area_increase result will be less so we return valid
key. Though in rtree_area_increase function we calculate the area
of the rectangle, so the result can be 'inf' if the rectangle is
huge enough, which is bigger than DBL_MAX.

Code of the rtree_pick_key modified so we always return a valid key.

into  trift2.:/MySQL/M50/push-5.0

added check for hidden I_S tables for 'show columns|keys' commands

into  production.mysql.com:/usersnfs/jperkin/bk/bug-27692/5.0

were accidentally removed during a previous rototill of this
code.  Fixes bug#27692.
  
While it can be argued we should strive to provide a 'secure by
default' installation, this happens to be the setup currently
documented in the manual as the default, so defer changes that
improve security out of the box to a co-ordinated effort later
on.
  
For now, make a note about the test databases and anonymous user
in mysql_install_db and recommend that mysql_secure_installation
be ran for users wishing to remove these defaults.

[..re-commit of previously lost change..]