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/parsenodes.h

@@ -1113,6 +1113,7 @@ typedef enum ObjectType
 	OBJECT_CONVERSION,
 	OBJECT_DATABASE,
 	OBJECT_DOMAIN,
+	OBJECT_EVENT_TRIGGER,
 	OBJECT_EXTENSION,
 	OBJECT_FDW,
 	OBJECT_FOREIGN_SERVER,
@@ -1731,6 +1732,32 @@ typedef struct CreateTrigStmt
 } CreateTrigStmt;
 
 /* ----------------------
+ *		Create EVENT TRIGGER Statement
+ * ----------------------
+ */
+typedef struct CreateEventTrigStmt
+{
+	NodeTag		type;
+	char	   *trigname;		/* TRIGGER's name */
+	char	   *eventname;		/* event's identifier */
+	List	   *whenclause;		/* list of DefElems indicating filtering */
+	List	   *funcname;		/* qual. name of function to call */
+} CreateEventTrigStmt;
+
+/* ----------------------
+ *		Alter EVENT TRIGGER Statement
+ * ----------------------
+ */
+typedef struct AlterEventTrigStmt
+{
+	NodeTag		type;
+	char	   *trigname;		/* TRIGGER's name */
+	char        tgenabled;		/* trigger's firing configuration WRT
+								 * session_replication_role */
+} AlterEventTrigStmt;
+
+/* ----------------------
+ *		Create/Drop PROCEDURAL LANGUAGE Statements
  *		Create PROCEDURAL LANGUAGE Statements
  * ----------------------
  */