Fixed bug#11764334 (formerly bug#57156): ALTER EVENT CHANGES

THE EVENT STATUS.

Any ALTER EVENT statement on a disabled event enabled it back
(unless this ALTER EVENT statement explicitly disabled the event).

The problem was that during processing of an ALTER EVENT statement
value of status field was overwritten unconditionally even if new
value was not specified explicitly. As a consequence this field
was set to default value for status which corresponds to ENABLE.

The solution is to check if status field was explicitly specified in
ALTER EVENT statement before assigning new value to status field.

mysql-test/r/events_bugs.result:
  test's result for Bug#11764334 was added.
mysql-test/t/events_bugs.test:
  new test for Bug#11764334 was added.
sql/event_db_repository.cc:
  mysql_event_fill_row() was modified: set value for status field
  in events tables only in case if statement CREATE EVENT
  is being processed or if this value was set in ALTER EVENT
  statement.
  Event_db_repository::create_event was modified: removed redundant
  setting of status field after return from call to mysql_event_fill_row().
sql/event_parse_data.h:
  Event_parse_data structure was modified: added flag
  status_changed that is set to true if status's value
  was changed in ALTER EVENT statement.
sql/sql_yacc.yy:
  Set flag status_changed if status was set in ALTER EVENT
  statement.

sql/event_parse_data.h

@@ -49,6 +49,7 @@ public:
 
   int on_completion;
   int status;
+  bool status_changed;
   longlong originator;
   /*
     do_not_create will be set if STARTS time is in the past and