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

include/libxml/HTMLparser.h

@@ -177,6 +177,7 @@ XMLPUBFUN void XMLCALL
  */
 typedef enum {
     HTML_PARSE_RECOVER  = 1<<0, /* Relaxed parsing */
+    HTML_PARSE_NODEFDTD = 1<<2, /* do not default a doctype if not found */
     HTML_PARSE_NOERROR	= 1<<5,	/* suppress error reports */
     HTML_PARSE_NOWARNING= 1<<6,	/* suppress warning reports */
     HTML_PARSE_PEDANTIC	= 1<<7,	/* pedantic error reporting */