WL #1034 (Internal CRON)


mysql-test/r/events.result:
  fix the results
mysql-test/t/disabled.def:
  make it enabled
mysql-test/t/events.test:
  - comment out some tests (having problems now)
  - add test for fix for bug#16548
sql/sql_yacc.yy:
  - fix for bug#16548 (Events: can't alter clause for preserve)
  - prefix optional non-terminals with opt_