There was already support for CREATE DEFINER=... EVENT syntax in the
parser, but DEFINER information was ignored.

This patch adds processing of DEFINER, and a new ALTER DEFINER=...
EVENT syntax.


mysql-test/r/events_bugs.result:
  Add result for bug#16425: Events: no DEFINER clause.
mysql-test/t/events_bugs.test:
  Add test case for bug#16425: Events: no DEFINER clause.
sql/event_data_objects.cc:
  Event_parse_data::init_definer() looks for DEFINER in
  thd->lex->definer, which is always set now.
sql/sql_parse.cc:
  Move DEFINER processing into the sp_process_definer().  Call this
  function for CREATE EVENT/ALTER EVENT, as well as for CREATE
  PROCEDURE/FUNCTION.
sql/sql_yacc.yy:
  Add 'alter DEFINER=... event', update rule references accordingly.