Add an id attributes to the sect1 tags that does not have one already.

This fixes filenames like x31.html, which is generated in html-split
mode, that somebody will end up linking to at some point.

en_US.ISO8859-1/articles/ipsec-must/article.sgml

@@ -40,7 +40,7 @@
     </abstract>
   </articleinfo>
 
-  <sect1>
+  <sect1 id="problem">
     <title>The Problem</title>
 
     <para>First, let's assume you have <link linkend="ipsec-install">
@@ -51,7 +51,7 @@
       But can you independently confirm it?</para>
   </sect1>
 
-  <sect1>
+  <sect1 id="solution">
     <title>The Solution</title>
 
     <para>First, some crypto-relevant info theory:</para>
@@ -107,7 +107,7 @@
     </sect2>
   </sect1>
 
-  <sect1>
+  <sect1 id="experiment">
     <title>The Experiment</title>
 
     <para>Here is the experiment:</para>