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.

src/include/utils/syscache.h

@@ -54,6 +54,8 @@ enum SysCacheIdentifier
 	DEFACLROLENSPOBJ,
 	ENUMOID,
 	ENUMTYPOIDNAME,
+	EVENTTRIGGERNAME,
+	EVENTTRIGGEROID,
 	FOREIGNDATAWRAPPERNAME,
 	FOREIGNDATAWRAPPEROID,
 	FOREIGNSERVERNAME,