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/createuser.sgml

@@ -49,7 +49,7 @@ PostgreSQL documentation
 
   <para>
    <application>createuser</application> is a wrapper around the
-   <acronym>SQL</acronym> command <xref linkend="sql-createrole">.
+   <acronym>SQL</acronym> command <xref linkend="sql-createrole"/>.
    There is no effective difference between creating users via
    this utility and via other methods for accessing the server.
   </para>
@@ -274,7 +274,7 @@ PostgreSQL documentation
        <para>
         The new user will have the <literal>REPLICATION</literal> privilege,
         which is described more fully in the documentation for <xref
-        linkend="sql-createrole">.
+        linkend="sql-createrole"/>.
        </para>
       </listitem>
      </varlistentry>
@@ -285,7 +285,7 @@ PostgreSQL documentation
        <para>
         The new user will not have the <literal>REPLICATION</literal>
         privilege, which is described more fully in the documentation for <xref
-        linkend="sql-createrole">.
+        linkend="sql-createrole"/>.
        </para>
       </listitem>
      </varlistentry>
@@ -405,7 +405,7 @@ PostgreSQL documentation
   <para>
    This utility, 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>
 
  </refsect1>
@@ -415,8 +415,8 @@ PostgreSQL documentation
   <title>Diagnostics</title>
 
   <para>
-   In case of difficulty, see <xref linkend="sql-createrole">
-   and <xref linkend="app-psql"> for
+   In case of difficulty, see <xref linkend="sql-createrole"/>
+   and <xref linkend="app-psql"/> for
    discussions of potential problems and error messages.
    The database server must be running at the
    targeted host.  Also, any default connection settings and environment
@@ -479,8 +479,8 @@ PostgreSQL documentation
   <title>See Also</title>
 
   <simplelist type="inline">
-   <member><xref linkend="app-dropuser"></member>
-   <member><xref linkend="sql-createrole"></member>
+   <member><xref linkend="app-dropuser"/></member>
+   <member><xref linkend="sql-createrole"/></member>
   </simplelist>
  </refsect1>