streamline the event worker thread work flow and try to eliminate
possibilities for memory corruptions that might have been
lurking in previous (complicated) code.
This patch: 
 * removes Event_job_data::compile that was never used
 * cleans up Event_job_data::execute to minimize juggling with
   thread context and eliminate unneded code paths
 * Implements Security_context::change/restore_security_context
   to be able to re-use these methods in all stored programs
This is to maybe fix Bug#27733 "Valgrind failures in 
remove_table_from_cache".
Review comments applied.

into  naruto.:C:/cpp/mysql-5.1-runtime

into  whalegate.ndb.mysql.com:/home/tomas/mysql-5.1

into  whalegate.ndb.mysql.com:/home/tomas/mysql-5.1-single-user

into  whalegate.ndb.mysql.com:/home/tomas/mysql-5.0-ndb

into  vajra.(none):/opt/local/work/mysql-5.1-runtime

This will hopefully fix events.test failure on vmware-win32, 
where scheduling is very primitive.

into  whalegate.ndb.mysql.com:/home/tomas/mysql-5.1-single-user

into  whalegate.ndb.mysql.com:/home/tomas/mysql-5.1-single-user

into  vajra.(none):/opt/local/work/mysql-5.1-runtime

Apparently it's the only platform in pushbuild where we 
compile without openssl.

- Giving the directive '--all-databases' to mysqldump caused 
  an attempt to lock and dump log tables which don't support this
  operation.
- With this patch the log tables are excluded from the set of
  databases tables to dump.

into  vajra.(none):/opt/local/work/mysql-5.1-c1

Bug#23631 "Events: SHOW VARIABLES doesn't work when mysql.event 
is damaged:

into  siva.hindu.god:/home/tsmith/m/bk/maint/51

  Bug #26591  Cluster handler does not set bits in null bytes correctly: Improved comments
ha_ndbcluster_binlog.cc:
  Bug #26591  Cluster handler does not set bits in null bytes correctly: Using empty_record() instead of bzero

into  vajra.(none):/opt/local/work/mysql-5.1-c1

when there are no up-to-date system tables to support it:
 - initialize the scheduler before reporting "Ready for connections".
   This ensures that warnings, if any, are printed before "Ready for
   connections", and this message is not mangled.
 - do not abort the scheduler if there are no system tables
 - check the tables once at start up, remember the status and disable
   the scheduler if the tables are not up to date.
   If one attempts to use the scheduler with bad tables,
   issue an error message.
 - clean up the behaviour of the module under LOCK TABLES and pre-locking
   mode
 - make sure implicit commit of Events DDL works as expected.
 - add more tests


Collateral clean ups in the events code.

This patch fixes Bug#23631 Events: SHOW VARIABLES doesn't work 
when mysql.event is damaged

into  whalegate.ndb.mysql.com:/home/tomas/mysql-5.1-single-user

into  whalegate.ndb.mysql.com:/home/tomas/mysql-5.0-ndb

into  whalegate.ndb.mysql.com:/home/tomas/mysql-5.1-single-user

into  whalegate.ndb.mysql.com:/home/tomas/mysql-5.1-single-user

into  whalegate.ndb.mysql.com:/home/tomas/mysql-5.0-ndb

into  whalegate.ndb.mysql.com:/home/tomas/mysql-5.1-single-user

into  mysql.com:/windows/Linux_space/MySQL/mysql-5.0-ndb

into  whalegate.ndb.mysql.com:/home/tomas/mysql-5.0-ndb

into  whalegate.ndb.mysql.com:/home/tomas/mysql-5.1-single-user

into  whalegate.ndb.mysql.com:/home/tomas/mysql-5.1-single-user

- test case workaround to avoid random failures