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_receivewal.sgml

@@ -36,15 +36,15 @@ PostgreSQL documentation
    log is streamed using the streaming replication protocol, and is written
    to a local directory of files. This directory can be used as the archive
    location for doing a restore using point-in-time recovery (see
-   <xref linkend="continuous-archiving">).
+   <xref linkend="continuous-archiving"/>).
   </para>
 
   <para>
    <application>pg_receivewal</application> streams the write-ahead
    log in real time as it's being generated on the server, and does not wait
-   for segments to complete like <xref linkend="guc-archive-command"> does.
+   for segments to complete like <xref linkend="guc-archive-command"/> does.
    For this reason, it is not necessary to set
-   <xref linkend="guc-archive-timeout"> when using
+   <xref linkend="guc-archive-timeout"/> when using
     <application>pg_receivewal</application>.
   </para>
 
@@ -60,9 +60,9 @@ PostgreSQL documentation
    <productname>PostgreSQL</productname> connection and uses the replication
    protocol. The connection must be made with a superuser or a user
    having <literal>REPLICATION</literal> permissions (see
-   <xref linkend="role-attributes">), and <filename>pg_hba.conf</filename>
+   <xref linkend="role-attributes"/>), and <filename>pg_hba.conf</filename>
    must permit the replication connection. The server must also be
-   configured with <xref linkend="guc-max-wal-senders"> set high enough to
+   configured with <xref linkend="guc-max-wal-senders"/> set high enough to
    leave at least one session available for the stream.
   </para>
 
@@ -172,7 +172,7 @@ PostgreSQL documentation
       <listitem>
         <para>
          Require <application>pg_receivewal</application> to use an existing
-         replication slot (see <xref linkend="streaming-replication-slots">).
+         replication slot (see <xref linkend="streaming-replication-slots"/>).
          When this option is used, <application>pg_receivewal</application> will report
          a flush position to the server, indicating when each segment has been
          synchronized to disk so that the server can remove that segment if it
@@ -244,7 +244,7 @@ PostgreSQL documentation
       <listitem>
        <para>
         Specifies parameters used to connect to the server, as a connection
-        string. See <xref linkend="libpq-connstring"> for more information.
+        string. See <xref linkend="libpq-connstring"/> for more information.
        </para>
        <para>
         The option is called <literal>--dbname</literal> for consistency with other
@@ -405,7 +405,7 @@ PostgreSQL documentation
   <para>
    This utility, like most other <productname>PostgreSQL</productname> utilities,
    uses the environment variables supported by <application>libpq</application>
-   (see <xref linkend="libpq-envars">).
+   (see <xref linkend="libpq-envars"/>).
   </para>
 
  </refsect1>
@@ -415,11 +415,11 @@ PostgreSQL documentation
 
   <para>
    When using <application>pg_receivewal</application> instead of
-   <xref linkend="guc-archive-command"> as the main WAL backup method, it is
+   <xref linkend="guc-archive-command"/> as the main WAL backup method, it is
    strongly recommended to use replication slots.  Otherwise, the server is
    free to recycle or remove write-ahead log files before they are backed up,
    because it does not have any information, either
-   from <xref linkend="guc-archive-command"> or the replication slots, about
+   from <xref linkend="guc-archive-command"/> or the replication slots, about
    how far the WAL stream has been archived.  Note, however, that a
    replication slot will fill up the server's disk space if the receiver does
    not keep up with fetching the WAL data.
@@ -443,7 +443,7 @@ PostgreSQL documentation
   <title>See Also</title>
 
   <simplelist type="inline">
-   <member><xref linkend="app-pgbasebackup"></member>
+   <member><xref linkend="app-pgbasebackup"/></member>
   </simplelist>
  </refsect1>