Add GUC for temporarily disabling event triggers

In order to troubleshoot misbehaving or buggy event triggers, the
documented advice is to enter single-user mode.  In an attempt to
reduce the number of situations where single-user mode is required
(or even recommended) for non-extraordinary maintenance, this GUC
allows to temporarily suspend event triggers.

This was originally extracted from a larger patchset which aimed
at supporting event triggers on login events.

Reviewed-by: Ted Yu <yuzhihong@gmail.com>
Reviewed-by: Mikhail Gribkov <youzhick@gmail.com>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/9140106E-F9BF-4D85-8FC8-F2D3C094A6D9@yesql.se
Discussion: https://postgr.es/m/0d46d29f-4558-3af9-9c85-7774e14a7709@postgrespro.ru

doc/src/sgml/ref/create_event_trigger.sgml

@@ -121,9 +121,12 @@ CREATE EVENT TRIGGER <replaceable class="parameter">name</replaceable>
 
   <para>
    Event triggers are disabled in single-user mode (see <xref
-   linkend="app-postgres"/>).  If an erroneous event trigger disables the
-   database so much that you can't even drop the trigger, restart in
-   single-user mode and you'll be able to do that.
+   linkend="app-postgres"/>) as well as when
+   <xref linkend="guc-event-triggers"/> is set to <literal>false</literal>.
+   If an erroneous event trigger disables the database so much that you can't
+   even drop the trigger, restart with <xref linkend="guc-event-triggers"/>
+   set to <literal>false</literal> to temporarily disable event triggers, or
+   in single-user mode, and you'll be able to do that.
   </para>
  </refsect1>