- now when the event is created and STARTS is omitted then STARTS is implicitly
  CURRENT_TIMESTAMP
- This CS also fixed incorrect presentation of STARTS/ENDS in I_S.EVENTS
(incorporated review changes)


mysql-test/r/events.result:
  results of new test cases
mysql-test/t/events.test:
  new test cases for bug #16537 (Events: mysql.event.starts is null)
sql/event.cc:
  - check whether event_timed::starts_null only in case
    event_timed::expression is set, so for recurring events only
  - disable binlogging of CREATE EVENT statement. It should not be
    replicated but the result of the execution. Still the replication is
    untouched topic.
sql/event.h:
  - add flags whether starts, ends and execute_at are null or not
sql/event_executor.cc:
  - check whether xxx_null instead of !xxxx.year
sql/event_timed.cc:
  - introduce xxx_null and change the usage of xxx.year to !xxx_null
sql/sql_show.cc:
  - don't show 0000-00-00 in I_S.EVENTS when the value is NULL
sql/sql_yacc.yy:
  - if STARTS is omitted default to current_timestamp