disabled, deferred triggers

This trivial patch implements disabled, deferred triggers, per my
proposal on -hackers a couple days ago. The 'tgenabled' status of the
trigger is consulted when the trigger is added to the deferred queue
(i.e. when the event occurs that fires the trigger), not when the
deferred event is executed.

No regression tests, as another bug prevents them (the pg_trigger row
for a trigger is only loaded once per session, so any changes to it are
not noticed unless the client disconnects and reconnects).

Cheers,

Neil
--
Neil Conway <neilc(at)samurai(dot)com> || PGP Key ID: DB3C29FC