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/pg_ctl-ref.sgml

@@ -140,7 +140,7 @@ PostgreSQL documentation
    <application>pg_ctl</application> is a utility for initializing a
    <productname>PostgreSQL</productname> database cluster, starting,
    stopping, or restarting the <productname>PostgreSQL</productname>
-   database server (<xref linkend="app-postgres">), or displaying the
+   database server (<xref linkend="app-postgres"/>), or displaying the
    status of a running server.  Although the server can be started
    manually, <application>pg_ctl</application> encapsulates tasks such
    as redirecting log output and properly detaching from the terminal
@@ -153,7 +153,7 @@ PostgreSQL documentation
    <productname>PostgreSQL</productname> database cluster, that is,
    a collection of databases that will be managed by a single
    server instance.  This mode invokes the <command>initdb</command>
-   command.  See <xref linkend="app-initdb"> for details.
+   command.  See <xref linkend="app-initdb"/> for details.
   </para>
 
   <para>
@@ -475,7 +475,7 @@ PostgreSQL documentation
        default is <literal>PostgreSQL</literal>.  Note that this only controls
        messages sent from <application>pg_ctl</application> itself; once
        started, the server will use the event source specified
-       by its <xref linkend="guc-event-source"> parameter.  Should the server
+       by its <xref linkend="guc-event-source"/> parameter.  Should the server
        fail very early in startup, before that parameter has been set,
        it might also log using the default event
        source name <literal>PostgreSQL</literal>.
@@ -567,12 +567,12 @@ PostgreSQL documentation
    <command>pg_ctl</command>, like most other <productname>PostgreSQL</productname>
    utilities,
    also uses the environment variables supported by <application>libpq</application>
-   (see <xref linkend="libpq-envars">).
+   (see <xref linkend="libpq-envars"/>).
   </para>
 
   <para>
    For additional variables that affect the server,
-   see <xref linkend="app-postgres">.
+   see <xref linkend="app-postgres"/>.
   </para>
  </refsect1>
 
@@ -691,8 +691,8 @@ pg_ctl: server is running (PID: 13718)
   <title>See Also</title>
 
   <simplelist type="inline">
-   <member><xref linkend="app-initdb"></member>
-   <member><xref linkend="app-postgres"></member>
+   <member><xref linkend="app-initdb"/></member>
+   <member><xref linkend="app-postgres"/></member>
   </simplelist>
  </refsect1>