Made sure webstructure.xml API produces valid XML.

Host names should not contain XML special characters such as quotation
mark, but at this stage the WebGraph may have mistakenly recorded a host
name with such characters. What's more the DigestURL constructor does
not prevent this.
By the way using serverObjects.putXML to encode host names we ensure
here the rendered XML is well formed and can be parsed by external tools
even if an structure entry is incorrect.

htroot/api/webstructure.java

@@ -263,7 +263,7 @@ public class webstructure {
 
     public static void reference(serverObjects prop, String prefix, int c, WebStructureGraph.StructureEntry sentry, WebStructureGraph ws) {
         prop.put(prefix + "_domains_" + c + "_hash", sentry.hosthash);
-        prop.put(prefix + "_domains_" + c + "_domain", sentry.hostname);
+        prop.putXML(prefix + "_domains_" + c + "_domain", sentry.hostname);
         prop.put(prefix + "_domains_" + c + "_date", sentry.date);
         Iterator<Map.Entry<String, Integer>> k = sentry.references.entrySet().iterator();
         Map.Entry<String, Integer> refentry;
@@ -276,7 +276,7 @@ public class webstructure {
             refdom = ws.hostHash2hostName(refhash);
             if (refdom == null) continue refloop;
             prop.put(prefix + "_domains_" + c + "_citations_" + d + "_refhash", refhash);
-            prop.put(prefix + "_domains_" + c + "_citations_" + d + "_refdom", refdom);
+            prop.putXML(prefix + "_domains_" + c + "_citations_" + d + "_refdom", refdom);
             refcount = refentry.getValue();
             prop.put(prefix + "_domains_" + c + "_citations_" + d + "_refcount", refcount.intValue());
             d++;