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/plperl.sgml

@@ -41,7 +41,7 @@
    <para>
     Users of source packages must specially enable the build of
     PL/Perl during the installation process.  (Refer to <xref
-    linkend="installation"> for more information.)  Users of
+    linkend="installation"/> for more information.)  Users of
     binary packages might find PL/Perl in a separate subpackage.
    </para>
   </note>
@@ -51,7 +51,7 @@
 
   <para>
    To create a function in the PL/Perl language, use the standard
-   <xref linkend="sql-createfunction">
+   <xref linkend="sql-createfunction"/>
    syntax:
 
 <programlisting>
@@ -69,7 +69,7 @@ $$ LANGUAGE plperl;
 
   <para>
    PL/Perl also supports anonymous code blocks called with the
-   <xref linkend="sql-do"> statement:
+   <xref linkend="sql-do"/> statement:
 
 <programlisting>
 DO $$
@@ -99,11 +99,11 @@ $$ LANGUAGE plperl;
    The syntax of the <command>CREATE FUNCTION</command> command requires
    the function body to be written as a string constant.  It is usually
    most convenient to use dollar quoting (see <xref
-   linkend="sql-syntax-dollar-quoting">) for the string constant.
+   linkend="sql-syntax-dollar-quoting"/>) for the string constant.
    If you choose to use escape string syntax <literal>E''</literal>,
    you must double any single quote marks (<literal>'</literal>) and backslashes
    (<literal>\</literal>) used in the body of the function
-   (see <xref linkend="sql-syntax-strings">).
+   (see <xref linkend="sql-syntax-strings"/>).
   </para>
 
   <para>
@@ -686,9 +686,9 @@ SELECT release_hosts_query();
         priority levels.
         Whether messages of a particular priority are reported to the client,
         written to the server log, or both is controlled by the
-        <xref linkend="guc-log-min-messages"> and
-        <xref linkend="guc-client-min-messages"> configuration
-        variables. See <xref linkend="runtime-config"> for more
+        <xref linkend="guc-log-min-messages"/> and
+        <xref linkend="guc-client-min-messages"/> configuration
+        variables. See <xref linkend="runtime-config"/> for more
         information.
       </para>
      </listitem>
@@ -792,7 +792,7 @@ SELECT release_hosts_query();
      <listitem>
       <para>
         Returns the contents of the referenced array as a string in array literal format
-        (see <xref linkend="arrays-input">).
+        (see <xref linkend="arrays-input"/>).
         Returns the argument value unaltered if it's not a reference to an array.
         The delimiter used between elements of the array literal defaults to "<literal>, </literal>"
         if a delimiter is not specified or is undef.
@@ -828,7 +828,7 @@ SELECT release_hosts_query();
      <listitem>
       <para>
         Returns the contents of the referenced array as a string in array constructor format
-        (see <xref linkend="sql-syntax-array-constructors">).
+        (see <xref linkend="sql-syntax-array-constructors"/>).
         Individual values are quoted using <function>quote_nullable</function>.
         Returns the argument value, quoted using <function>quote_nullable</function>,
         if it's not a reference to an array.
@@ -1336,7 +1336,7 @@ DO 'elog(WARNING, join ", ", sort keys %INC)' LANGUAGE plperl;
        </para>
        <para>
         Initialization will happen in the postmaster if the <literal>plperl</literal> library is
-        included in <xref linkend="guc-shared-preload-libraries">, in which
+        included in <xref linkend="guc-shared-preload-libraries"/>, in which
         case extra consideration should be given to the risk of destabilizing
         the postmaster.  The principal reason for making use of this feature
         is that Perl modules loaded by <literal>plperl.on_init</literal> need be