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/sepgsql.sgml

@@ -17,7 +17,7 @@
    <para>
      The current implementation has significant limitations, and does not
      enforce mandatory access control for all actions.  See
-     <xref linkend="sepgsql-limitations">.
+     <xref linkend="sepgsql-limitations"/>.
    </para>
  </warning>
 
@@ -51,7 +51,7 @@
   </para>
 
   <para>
-   The <xref linkend="sql-security-label"> statement allows assignment of
+   The <xref linkend="sql-security-label"/> statement allows assignment of
    a security label to a database object.
   </para>
 
@@ -93,7 +93,7 @@ Policy from config file:        targeted
 
   <para>
    To use this module, you must include <literal>sepgsql</literal>
-   in the <xref linkend="guc-shared-preload-libraries"> parameter in
+   in the <xref linkend="guc-shared-preload-libraries"/> parameter in
    <filename>postgresql.conf</filename>.  The module will not function correctly
    if loaded in any other manner.  Once the module is loaded, you
    should execute <filename>sepgsql.sql</filename> in each database.
@@ -161,7 +161,7 @@ $ for DBNAME in template0 template1 postgres; do
 
   <para>
    First, set up <filename>sepgsql</filename> in a working database
-   according to the instructions in <xref linkend="sepgsql-installation">.
+   according to the instructions in <xref linkend="sepgsql-installation"/>.
    Note that the current operating system user must be able to connect to the
    database as superuser without password authentication.
   </para>
@@ -215,7 +215,7 @@ unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
 </screen>
 
   <para>
-   See <xref linkend="sepgsql-resources"> for details on adjusting your
+   See <xref linkend="sepgsql-resources"/> for details on adjusting your
    working domain, if necessary.
   </para>
 
@@ -448,7 +448,7 @@ UPDATE t1 SET x = 2, y = md5sum(y) WHERE z = 100;
    <itemizedlist>
     <listitem>
      <para>
-      <xref linkend="sql-createdatabase"> additionally requires
+      <xref linkend="sql-createdatabase"/> additionally requires
       <literal>getattr</literal> permission for the source or template database.
      </para>
     </listitem>
@@ -506,7 +506,7 @@ UPDATE t1 SET x = 2, y = md5sum(y) WHERE z = 100;
     </listitem>
     <listitem>
      <para>
-      Using <xref linkend="sql-security-label"> on an object additionally
+      Using <xref linkend="sql-security-label"/> on an object additionally
       requires <literal>relabelfrom</literal> permission for the object in
       conjunction with its old security label and <literal>relabelto</literal>
       permission for the object in conjunction with its new security label.
@@ -641,7 +641,7 @@ ERROR:  SELinux: security policy violation
   <sect3>
    <title>Miscellaneous</title>
    <para>
-    We reject the <xref linkend="sql-load"> command across the board, because
+    We reject the <xref linkend="sql-load"/> command across the board, because
     any module loaded could easily circumvent security policy enforcement.
    </para>
 
@@ -651,7 +651,7 @@ ERROR:  SELinux: security policy violation
  <sect2 id="sepgsql-functions">
   <title>Sepgsql Functions</title>
   <para>
-   <xref linkend="sepgsql-functions-table"> shows the available functions.
+   <xref linkend="sepgsql-functions-table"/> shows the available functions.
   </para>
 
   <table id="sepgsql-functions-table">