Convert documentation to DocBook XML

Since some preparation work had already been done, the only source
changes left were changing empty-element tags like <xref linkend="foo">
to <xref linkend="foo"/>, and changing the DOCTYPE.

The source files are still named *.sgml, but they are actually XML files
now.  Renaming could be considered later.

In the build system, the intermediate step to convert from SGML to XML
is removed.  Everything is build straight from the source files again.
The OpenSP (or the old SP) package is no longer needed.

The documentation toolchain instructions are updated and are much
simpler now.

Peter Eisentraut, Alexander Lakhin, Jürgen Purtz

doc/src/sgml/ref/create_event_trigger.sgml

@@ -36,7 +36,7 @@ CREATE EVENT TRIGGER <replaceable class="parameter">name</replaceable>
    Whenever the designated event occurs and the <literal>WHEN</literal> condition
    associated with the trigger, if any, is satisfied, the trigger function
    will be executed.  For a general introduction to event triggers, see
-   <xref linkend="event-triggers">.  The user who creates an event trigger
+   <xref linkend="event-triggers"/>.  The user who creates an event trigger
    becomes its owner.
   </para>
  </refsect1>
@@ -60,7 +60,7 @@ CREATE EVENT TRIGGER <replaceable class="parameter">name</replaceable>
     <listitem>
      <para>
       The name of the event that triggers a call to the given function.
-      See <xref linkend="event-trigger-definition"> for more information
+      See <xref linkend="event-trigger-definition"/> for more information
       on event names.
      </para>
     </listitem>
@@ -113,7 +113,7 @@ 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
+   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.
   </para>
@@ -154,9 +154,9 @@ CREATE EVENT TRIGGER abort_ddl ON ddl_command_start
   <title>See Also</title>
 
   <simplelist type="inline">
-   <member><xref linkend="sql-altereventtrigger"></member>
-   <member><xref linkend="sql-dropeventtrigger"></member>
-   <member><xref linkend="sql-createfunction"></member>
+   <member><xref linkend="sql-altereventtrigger"/></member>
+   <member><xref linkend="sql-dropeventtrigger"/></member>
+   <member><xref linkend="sql-createfunction"/></member>
   </simplelist>
  </refsect1>
 </refentry>