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/bin/psql/command.c

@@ -490,6 +490,9 @@ exec_command(const char *cmd,
 				else
 					success = listExtensions(pattern);
 				break;
+			case 'y':			/* Event Triggers */
+				success = listEventTriggers(pattern, show_verbose);
+				break;
 			default:
 				status = PSQL_CMD_UNKNOWN;
 		}