Allow mysql_upgrade to enable event after table is corrected

new features:
set event_scheduler=ON|OFF will now try to init event scheduler
if it's not enabled
set event_scheduler=default will try to enable it based on
the value of the event_scheduler when mysqld was started

mysql-test/t/events_restart.test

@@ -71,9 +71,9 @@ drop event intact_check_1;
 drop event intact_check_2;
 --error ER_EVENTS_DB_ERROR
 drop event intact_check;
---error ER_EVENTS_DB_ERROR
+--error ER_STARTUP
 set global event_scheduler=on;
---error ER_EVENTS_DB_ERROR
+--error ER_STARTUP
 set global event_scheduler=off;
 show variables like 'event_scheduler';
 --echo Make sure that we still can create and drop databases,
@@ -84,6 +84,16 @@ drop database mysqltest_db1;
 --echo Restore the original mysql.event table
 drop table mysql.event;
 rename table event_like to mysql.event;
+
+--echo check that we can now enable events without restart
+set global event_scheduler=original;
+select @@global.event_scheduler;
+set global event_scheduler=on;
+select @@global.event_scheduler;
+--sorted_result
+--replace_column 6 # 9 # 10 #
+show events;
+
 --echo Now let's restart the server again
 
 --source include/restart_mysqld.inc

mysql-test/t/mysql_upgrade-6984.test

@@ -19,4 +19,5 @@ connect(con1,localhost,root,foo,,,);
 
 update mysql.user set password='' where user='root';
 flush privileges;
-
+# Load event table
+set global event_scheduler=OFF;

mysql-test/t/skip_grants.test

@@ -112,8 +112,8 @@ DROP FUNCTION f3;
 #
 # Bug #26807 "set global event_scheduler=1" and --skip-grant-tables crashes server
 #
---error ER_OPTION_PREVENTS_STATEMENT
 set global event_scheduler=1;
+set global event_scheduler=0;
 
 #
 # Bug#26285 Selecting information_schema crahes server