WL#1032
sql_mode setting was disregarded during create/alter event and wasn't set during
event execution.
(post-review small fixes)


sql/event.cc:
  store the sql_mode when the body is stored
  - always on create event
  - during alter event if the user changes the body
sql/event.h:
  add sql_mode as member variable
sql/event_timed.cc:
  - exchange thd->variables.sql_mode before and after execution
  - set the sql_mode of the anonymous SP -> event_timed::sphead
sql/sql_show.cc:
  - show real content in I_S.EVENTS.SQL_MODE , disallow NULL