Fix pg_dump's handling of event triggers.

Fix pg_dump's handling of event triggers.

pg_dump with the --clean option failed to emit DROP EVENT TRIGGER
commands for event triggers.  In a closely related oversight,
it also did not emit ALTER OWNER commands for event triggers.
Since only superusers can create event triggers, the latter oversight
is of little practical consequence ... but if we're going to record
an owner for event triggers, then surely pg_dump should preserve it.

Per complaint from Greg Atkins.  Back-patch to 9.3 where event triggers
were introduced.

Discussion: https://postgr.es/m/20170722191142[email protected]