Before this change, the functions BENCHMARK, ENCODE, DECODE and FORMAT could
only accept a constant for some parameters.

After this change, this restriction has been removed. An implication is that
these functions can also be used in prepared statements.

The change consist of changing the following classes:
- Item_func_benchmark
- Item_func_encode
- Item_func_decode
- Item_func_format
to:
- only accept Item* in the constructor,
- and evaluate arguments during calls to val_xxx()
which fits the general design of all the other functions.

The 'TODO' items identified in item_create.cc during the work done for
Bug 21114 are addressed by this fix, as a natural consequence of aligning
the design.

In the 'func_str' test, a single very long test line involving an explain
extended select with many functions has been rewritten into multiple
separate tests, to improve maintainability.
The result of explain extended select decode(encode(...)) has changed,
since the encode and decode functions now print all their parameters.

into  alik.:/mnt/raid/alik/MySQL/devel/5.1-rt-merged-2

into  outpost.site:/home/cps/mysql/trees/5.1-runtime-bug9191

which got broken during refactoring.

into  outpost.site:/home/cps/mysql/trees/5.1-runtime-bug9191

into  outpost.site:/home/cps/mysql/trees/5.0-runtime-bug9191

into  neptunus.(none):/home/msvensson/mysql/mysql-5.1-new-maint

into  neptunus.(none):/home/msvensson/mysql/mysql-5.1-new-maint

into  neptunus.(none):/home/msvensson/mysql/mysql-5.0-maint

Fixed the tests for case insensitive platforms

into  neptunus.(none):/home/msvensson/mysql/mysql-5.1-new-maint

into  neptunus.(none):/home/msvensson/mysql/mysql-5.0-maint

into  neptunus.(none):/home/msvensson/mysql/mysql-5.1-new-maint

into  neptunus.(none):/home/msvensson/mysql/my50-m-stress

Use lowercase for "Too long dbname" test as it will be put ot lower on some platforms and we don't wan't to test that.

into  neptunus.(none):/home/msvensson/mysql/my50-m-stress

into  alik.:/mnt/raid/alik/MySQL/devel/5.1-rt-merged

into  neptunus.(none):/home/msvensson/mysql/mysql-5.0-maint

into  moonlight.intranet:/home/tomash/src/mysql_ab/mysql-5.1-cleanup

into  moonlight.intranet:/home/tomash/src/mysql_ab/mysql-5.1-cleanup

into  moonlight.intranet:/home/tomash/src/mysql_ab/mysql-5.0-cleanup

into  zippy.cornsilk.net:/home/cmiller/work/mysql/mysql-5.1-maint

into  zippy.cornsilk.net:/home/cmiller/work/mysql/mysql-5.0-maint

into  zippy.cornsilk.net:/home/cmiller/work/mysql/mysql-4.1-maint

into  shellback.(none):/home/msvensson/mysql/mysql-4.1-maint