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
