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/begin.sgml

@@ -38,8 +38,8 @@ BEGIN [ WORK | TRANSACTION ] [ <replaceable class="parameter">transaction_mode</
    <command>BEGIN</command> initiates a transaction block, that is,
    all statements after a <command>BEGIN</command> command will be
    executed in a single transaction until an explicit <xref
-   linkend="sql-commit"> or <xref
-   linkend="sql-rollback"> is given.
+   linkend="sql-commit"/> or <xref
+   linkend="sql-rollback"/> is given.
    By default (without <command>BEGIN</command>),
    <productname>PostgreSQL</productname> executes
    transactions in <quote>autocommit</quote> mode, that is, each
@@ -60,7 +60,7 @@ BEGIN [ WORK | TRANSACTION ] [ <replaceable class="parameter">transaction_mode</
   <para>
    If the isolation level, read/write mode, or deferrable mode is specified, the new
    transaction has those characteristics, as if
-   <xref linkend="sql-set-transaction">
+   <xref linkend="sql-set-transaction"/>
    was executed.
   </para>
  </refsect1>
@@ -81,7 +81,7 @@ BEGIN [ WORK | TRANSACTION ] [ <replaceable class="parameter">transaction_mode</
   </variablelist>
 
   <para>
-   Refer to <xref linkend="sql-set-transaction"> for information on the meaning
+   Refer to <xref linkend="sql-set-transaction"/> for information on the meaning
    of the other parameters to this statement.
   </para>
  </refsect1>
@@ -90,13 +90,13 @@ BEGIN [ WORK | TRANSACTION ] [ <replaceable class="parameter">transaction_mode</
   <title>Notes</title>
 
   <para>
-   <xref linkend="sql-start-transaction"> has the same functionality
+   <xref linkend="sql-start-transaction"/> has the same functionality
    as <command>BEGIN</command>.
   </para>
 
   <para>
-   Use <xref linkend="sql-commit"> or
-   <xref linkend="sql-rollback">
+   Use <xref linkend="sql-commit"/> or
+   <xref linkend="sql-rollback"/>
    to terminate a transaction block.
   </para>
 
@@ -104,7 +104,7 @@ BEGIN [ WORK | TRANSACTION ] [ <replaceable class="parameter">transaction_mode</
    Issuing <command>BEGIN</command> when already inside a transaction block will
    provoke a warning message.  The state of the transaction is not affected.
    To nest transactions within a transaction block, use savepoints
-   (see <xref linkend="sql-savepoint">).
+   (see <xref linkend="sql-savepoint"/>).
   </para>
 
   <para>
@@ -131,7 +131,7 @@ BEGIN;
   <para>
    <command>BEGIN</command> is a <productname>PostgreSQL</productname>
    language extension.  It is equivalent to the SQL-standard command
-   <xref linkend="sql-start-transaction">, whose reference page
+   <xref linkend="sql-start-transaction"/>, whose reference page
    contains additional compatibility information.
   </para>
 
@@ -152,10 +152,10 @@ BEGIN;
   <title>See Also</title>
 
   <simplelist type="inline">
-   <member><xref linkend="sql-commit"></member>
-   <member><xref linkend="sql-rollback"></member>
-   <member><xref linkend="sql-start-transaction"></member>
-   <member><xref linkend="sql-savepoint"></member>
+   <member><xref linkend="sql-commit"/></member>
+   <member><xref linkend="sql-rollback"/></member>
+   <member><xref linkend="sql-start-transaction"/></member>
+   <member><xref linkend="sql-savepoint"/></member>
   </simplelist>
  </refsect1>
 </refentry>