Make new event trigger facility actually do something.
 
Commit 3855968f328918b6cd1401dd11d109d471a54d40 added syntax, pg_dump,
psql support, and documentation, but the triggers didn't actually fire.
With this commit, they now do.  This is still a pretty basic facility
overall because event triggers do not get a whole lot of information
about what the user is trying to do unless you write them in C; and
there's still no option to fire them anywhere except at the very
beginning of the execution sequence, but it's better than nothing,
and a good building block for future work.
 
Along the way, add a regression test for ALTER LARGE OBJECT, since
testing of event triggers reveals that we haven't got one.
 
Dimitri Fontaine and Robert Haas

Syntax support and documentation for event triggers.
 
They don't actually do anything yet; that will get fixed in a
follow-on commit.  But this gets the basic infrastructure in place,
including CREATE/ALTER/DROP EVENT TRIGGER; support for COMMENT,
SECURITY LABEL, and ALTER EXTENSION .. ADD/DROP EVENT TRIGGER;
pg_dump and psql support; and documentation for the anticipated
initial feature set.
 
Dimitri Fontaine, with review and a bunch of additional hacking by me.
Thom Brown extensively reviewed earlier versions of this patch set,
but there's not a whole lot of that code left in this commit, as it
turns out.

...event triggers do not get a whole lot of information about what the user is trying to do unless you write them in C...

CREATE OR REPLACE FUNCTION snitch() RETURNS event_trigger AS $$
BEGIN
    RAISE NOTICE 'snitch: % %', tg_event, tg_tag;
END;
$$ LANGUAGE plpgsql;
 
CREATE EVENT TRIGGER snitch ON ddl_command_start EXECUTE PROCEDURE snitch();

$ create table z (i int4);
NOTICE:  snitch: ddl_command_start CREATE TABLE
CREATE TABLE
 
$ alter table z add column j int8;
NOTICE:  snitch: ddl_command_start ALTER TABLE
ALTER TABLE
 
$ create index q on z(j);
NOTICE:  snitch: ddl_command_start CREATE INDEX
CREATE INDEX
 
$ drop table z;
NOTICE:  snitch: ddl_command_start DROP TABLE
DROP TABLE