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.

doc/src/sgml/ref/psql-ref.sgml

@@ -1455,6 +1455,20 @@ testdb=>
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><literal>\dy[+] [ <link linkend="APP-PSQL-patterns"><replaceable class="parameter">pattern</replaceable></link> ]</literal></term>
+        <listitem>
+        <para>
+        Lists event triggers.
+        If <replaceable class="parameter">pattern</replaceable>
+        is specified, only those event triggers whose names match the pattern
+        are listed.
+        If <literal>+</literal> is appended to the command name, each object
+        is listed with its associated description.
+        </para>
+        </listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><literal>\e</literal> or <literal>\edit</> <literal> <optional> <replaceable class="parameter">filename</> </optional> <optional> <replaceable class="parameter">line_number</> </optional> </literal></term>