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/nodes/nodes.h

@@ -357,6 +357,8 @@ typedef enum NodeTag
 	T_CreateExtensionStmt,
 	T_AlterExtensionStmt,
 	T_AlterExtensionContentsStmt,
+	T_CreateEventTrigStmt,
+	T_AlterEventTrigStmt,
 
 	/*
 	 * TAGS FOR PARSE TREE NODES (parsenodes.h)