[github39] system_messages tool converted to DocBook 5.0

doc/version.ent.in

-<!ENTITY __VERSION__ "@PACKAGE_VERSION@">
+<!ENTITY keaversion "@PACKAGE_VERSION@">

tools/system_messages.cc

@@ -76,32 +76,31 @@ DictionaryType dictionary;
 /// this is output before anything else.
 const std::string FILE_HEADER =
 "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n\
-<!DOCTYPE book PUBLIC \"-//OASIS//DTD DocBook XML V4.2//EN\"\n\
-\"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd\" [\n\
-<!ENTITY mdash  \"&#x2014;\" >\n\
-<!ENTITY % version SYSTEM \"version.ent\">\n\
-%version;\n\
-]>\n\
+<!DOCTYPE book [\
+<!ENTITY % keaversion SYSTEM \"version.ent\">\
+%keaversion;\
+]>\
+\n\
 <!--\n\
      This XML document is generated using the system_messages tool\n\
      based on the .mes message files.\n\
 \n\
      Do not edit this file.\n\
 -->\n\
-<book>\n\
+<book xmlns=\"http://docbook.org/ns/docbook\" version=\"5.0\">\n\
   <?xml-stylesheet href=\"kea-guide.css\" type=\"text/css\"?>\n\
 \n\
-  <bookinfo>\n\
+  <info>\n\
     <title>Kea Messages Manual</title>\n\
 \n\
     <copyright>\n\
-      <year>2011-2017</year>\n\
+      <year>2011-2018</year>\n\
       <holder>Internet Systems Consortium, Inc. (\"ISC\")</holder>\n\
     </copyright>\n\
 \n\
     <abstract>\n\
       <para>\n\
-        This is the messages manual for Kea version &__VERSION__;.\n\
+        This is the messages manual for Kea version &keaversion;.\n\
             The most up-to-date version of this document, along with\n\
             other documents for Kea, can be found at\n\
         <ulink url=\"http://kea.isc.org/docs\"/>.\n\
@@ -109,8 +108,8 @@ const std::string FILE_HEADER =
     </abstract>\n\
 \n\
     <releaseinfo>This is the messages manual for Kea version\n\
-        &__VERSION__;.</releaseinfo>\n\
-  </bookinfo>\n\
+        &keaversion;.</releaseinfo>\n\
+  </info>\n\
 \n\
   <chapter id=\"intro\">\n\
     <title>Introduction</title>\n\