updated sources


sql/event.cc:
  update
  put some error calls to the places they occur
sql/event.h:
  - change the default (does not work in STRICT mode)
sql/event_executor.cc:
  move mutex initialization to evex_init_mutexes so init_events() can be reused when the
  main thread does not work and set global event_scheduler=1; (this will start the thread)
  The main thread is now visible with show processlist and can be killed.
sql/event_priv.h:
  don't use anymore SP for opening table
sql/event_timed.cc:
  don't use anymore SP routines for opening mysql.event
sql/mysqld.cc:
  shutdown_events() should be maximal at the end of the server because
  it destroys mutexes of EVEX. The call should not be in the main thread.
sql/set_var.cc:
  make sys_var_event_executor subclass sys_var_bool_ptr
  to overload ::update() method - needed to start a
  killed (non-running) evex main thread
sql/set_var.h:
  declare class sys_var_event_executor
sql/share/errmsg.txt:
  2 new messages