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/alter_default_privileges.sgml

@@ -106,7 +106,7 @@ REVOKE [ GRANT OPTION FOR ]
   </para>
 
   <para>
-   As explained under <xref linkend="sql-grant">,
+   As explained under <xref linkend="sql-grant"/>,
    the default privileges for any object type normally grant all grantable
    permissions to the object owner, and may grant some privileges to
    <literal>PUBLIC</literal> as well.  However, this behavior can be changed by
@@ -150,8 +150,8 @@ REVOKE [ GRANT OPTION FOR ]
       This parameter, and all the other parameters in
       <replaceable class="parameter">abbreviated_grant_or_revoke</replaceable>,
       act as described under
-      <xref linkend="sql-grant"> or
-      <xref linkend="sql-revoke">,
+      <xref linkend="sql-grant"/> or
+      <xref linkend="sql-revoke"/>,
       except that one is setting permissions for a whole class of objects
       rather than specific named objects.
      </para>
@@ -165,11 +165,11 @@ REVOKE [ GRANT OPTION FOR ]
   <title>Notes</title>
 
   <para>
-   Use <xref linkend="app-psql">'s <command>\ddp</command> command
+   Use <xref linkend="app-psql"/>'s <command>\ddp</command> command
    to obtain information about existing assignments of default privileges.
    The meaning of the privilege values is the same as explained for
    <command>\dp</command> under
-   <xref linkend="sql-grant">.
+   <xref linkend="sql-grant"/>.
   </para>
 
   <para>
@@ -226,8 +226,8 @@ ALTER DEFAULT PRIVILEGES FOR ROLE admin REVOKE EXECUTE ON FUNCTIONS FROM PUBLIC;
   <title>See Also</title>
 
   <simplelist type="inline">
-   <member><xref linkend="sql-grant"></member>
-   <member><xref linkend="sql-revoke"></member>
+   <member><xref linkend="sql-grant"/></member>
+   <member><xref linkend="sql-revoke"/></member>
   </simplelist>
  </refsect1>