Add an HTML parser option to avoid a default doctype

- include/libxml/HTMLparser.h: defines the new HTML parser option
  HTML_PARSE_NODEFDTD
- HTMLparser.c: if option is set don't add a default DTD
- xmllint.c: add the corresponding --nodefdtd option in xmllint
diff --git a/HTMLparser.c b/HTMLparser.c
index 42dc776..224c65f 100644
--- a/HTMLparser.c
+++ b/HTMLparser.c
@@ -4670,7 +4670,7 @@
     if ((ctxt->sax) && (ctxt->sax->endDocument != NULL))
         ctxt->sax->endDocument(ctxt->userData);
 
-    if (ctxt->myDoc != NULL) {
+    if ((!(ctxt->options & HTML_PARSE_NODEFDTD)) && (ctxt->myDoc != NULL)) {
 	dtd = xmlGetIntSubset(ctxt->myDoc);
 	if (dtd == NULL)
 	    ctxt->myDoc->intSubset =
@@ -6530,6 +6530,10 @@
 	ctxt->options |= XML_PARSE_HUGE;
         options -= XML_PARSE_HUGE;
     }
+    if (options & HTML_PARSE_NODEFDTD) {
+	ctxt->options |= HTML_PARSE_NODEFDTD;
+        options -= HTML_PARSE_NODEFDTD;
+    }
     ctxt->dictNames = 0;
     return (options);
 }