If [NOT] PRESERVE was not given, parser always defaulted to NOT
PRESERVE, making it impossible for the "not given = no change"
rule to work in ALTER EVENT. Leaving out the PRESERVE-clause
defaults to NOT PRESERVE on CREATE now, and to "no change" in
ALTER.

mysql-test/r/events_2.result:
  show that giving no PRESERVE-clause to ALTER EVENT
  results in no change. show that giving no PRESERVE-clause
  to CREATE EVENT defaults to NOT PRESERVE as per the docs.
  Show specifically that this is also handled correctly when
  trying to ALTER EVENTs into the past.
mysql-test/t/events_2.test:
  show that giving no PRESERVE-clause to ALTER EVENT
  results in no change. show that giving no PRESERVE-clause
  to CREATE EVENT defaults to NOT PRESERVE as per the docs.
  Show specifically that this is also handled correctly when
  trying to ALTER EVENTs into the past.
sql/event_db_repository.cc:
  If ALTER EVENT was given no PRESERVE-clause (meaning "no change"),
  we don't know the previous PRESERVE-setting by the time we check
  the parse-data. If ALTER EVENT was given dates that are in the past,
  we don't know how to react, lacking the PRESERVE-setting. Heal this
  by running the check later when we have actually read the previous
  EVENT-data.
sql/event_parse_data.cc:
  Change default for ON COMPLETION to indicate, "not specified."
  Also defer throwing errors when ALTER EVENT is given dates in
  the past but not PRESERVE-clause until we know the previous
  PRESERVE-value.
sql/event_parse_data.h:
  Add third state for ON COMPLETION [NOT] PRESERVE (preserve,
  don't, not specified).
  
  Make check_dates() public so we can defer this check until
  deeper in the callstack where we have all the required data.
sql/sql_yacc.yy:
  If CREATE EVENT is not given ON COMPLETION [NOT] PRESERVE,
  we default to NOT, as per the docs.