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/reference.sgml

@@ -41,6 +41,7 @@
    &alterDefaultPrivileges;
    &alterDomain;
    &alterExtension;
+   &alterEventTrigger;
    &alterForeignDataWrapper;
    &alterForeignTable;
    &alterFunction;
@@ -82,6 +83,7 @@
    &createDatabase;
    &createDomain;
    &createExtension;
+   &createEventTrigger;
    &createForeignDataWrapper;
    &createForeignTable;
    &createFunction;
@@ -120,6 +122,7 @@
    &dropDatabase;
    &dropDomain;
    &dropExtension;
+   &dropEventTrigger;
    &dropForeignDataWrapper;
    &dropForeignTable;
    &dropFunction;