Make new event trigger facility actually do something.

Commit 3855968f32 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

doc/src/sgml/ref/create_event_trigger.sgml

@@ -98,12 +98,6 @@ CREATE EVENT TRIGGER <replaceable class="PARAMETER">name</replaceable>
       A user-supplied function that is declared as taking no argument and
       returning type <literal>event_trigger</literal>.
      </para>
-     <para>
-      If your event trigger is implemented in <literal>C</literal> then it
-      will be called with an argument, of
-      type <literal>internal</literal>, which is a pointer to
-      the <literal>Node *</literal> parse tree.
-     </para>
     </listitem>
    </varlistentry>