This change set implements the DROP TRIGGER IF EXISTS functionality.

This fix is considered a bug and not a feature, because without it,
there is no known method to write a database creation script that can create
a trigger without failing, when executed on a database that may or may not
contain already a trigger of the same name.

Implementing this functionality closes an orthogonality gap between triggers
and stored procedures / stored functions (which do support the DROP IF
EXISTS syntax).

In sql_trigger.cc, in mysql_create_or_drop_trigger,
the code has been reordered to:
- perform the tests that do not depend on the file system (access()),
- get the locks (wait_if_global_read_lock, LOCK_open)
- call access()
- perform the operation
- write to the binlog
- unlock (LOCK_open, start_waiting_global_read_lock)

This is to ensure that all the code that depends on the presence of the
trigger file is executed in the same critical section,
and prevents race conditions similar to the case fixed by Bug 14262 :

- thread 1 executes DROP TRIGGER IF EXISTS, access() returns a failure
- thread 2 executes CREATE TRIGGER
- thread 2 logs CREATE TRIGGER
- thread 1 logs DROP TRIGGER IF EXISTS

The patch itself is based on code contributed by the MySQL community,
under the terms of the Contributor License Agreement (See Bug 18161).

into  mockturtle.local:/home/dlenev/src/mysql-5.0-bg23651

stored function invoked from different connections".

Invocation of trigger which was using stored function from different
connections caused server crashes (for non-debug server this happened
in highly concurrent environment, but debug server failed on assertion
in relatively simple scenario).

Item_func_sp was not safe to use in triggers (in other words for
re-execution from different threads) as artificial TABLE object
pointed by Item_func_sp::dummy_table referenced incorrect THD
object. To fix the problem we force re-initialization of this
object for each re-execution of statement.

into  bodhi.local:/opt/local/work/mysql-5.0-runtime

into  mysql.com:/Users/kent/mysql/bk/mysql-5.0

  Test for base directory when a RPM install corrected

into  bodhi.local:/opt/local/work/mysql-5.0-runtime

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

into  quadxeon.mysql.com:/benchmarks/ext3/TOSAVE/tsmith/bk/50

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

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

into  trift2.:/MySQL/M41/push-4.1

into  trift2.:/MySQL/M41/push-4.1

into  kahlann.erinye.com:/home/df/mysql/build/mtr-4.1

into  kahlann.erinye.com:/home/df/mysql/build/mtr-5.0

into  kahlann.erinye.com:/home/df/mysql/build/mtr-5.0

into  outpost.site:/home/cps/mysql/trees/4.1-runtime-bug9191

into  mysql.com:/Users/kent/mysql/bk/mysql-5.0

into  mysql.com:/Users/kent/mysql/bk/lic/my50-lic

  Added --force-restart pseudo server argument to shell version as well

    Adapt to the different path of the server program, if installation was by RPM.
    Missing in the first changeset for bug#17194.

into  quadxeon.mysql.com:/benchmarks/ext3/TOSAVE/tsmith/bk/inno/50

Fixes:
- Bug #24089: Race condition in fil_flush_file_spaces()

Fixes:
- Bug #15815: Very poor performance with multiple queries running concurrently
- Bug #22868: 'Thread thrashing' with > 50 concurrent conns under an upd-intensive workloadw
- Bug #23769: Debug assertion failure with innodb_locks_unsafe_for_binlog
- Bug #24089: Race condition in fil_flush_file_spaces()

into  quadxeon.mysql.com:/benchmarks/ext3/TOSAVE/tsmith/bk/50

into  quadxeon.mysql.com:/benchmarks/ext3/TOSAVE/tsmith/bk/41

into  quadxeon.mysql.com:/benchmarks/ext3/TOSAVE/tsmith/bk/50

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

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

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

Pass location of charsets dir to mysqltest to avoid using the charsetdir locally installed on machine. 

into  trift2.:/MySQL/M50/bug24023-5.0

into  trift2.:/MySQL/M41/clone-4.1

into  trift2.:/MySQL/M41/clone-4.1