Rework validation context flags

Use a bitmask instead of magic values to

- keep track whether the validation context is part of a parser context
- keep track whether xmlValidateDtdFinal was called

This allows to add addtional flags later.

Note that this deliberately changes the name of a public struct member,
assuming that this was always private data never to be used by client
code.

HTMLparser.c

@@ -5118,7 +5118,7 @@ htmlInitParserCtxt(htmlParserCtxtPtr ctxt)
     ctxt->linenumbers = xmlLineNumbersDefaultValue;
     ctxt->keepBlanks = xmlKeepBlanksDefaultValue;
     ctxt->html = 1;
-    ctxt->vctxt.finishDtd = XML_CTXT_FINISH_DTD_0;
+    ctxt->vctxt.flags = XML_VCTXT_USE_PCTXT;
     ctxt->vctxt.userData = ctxt;
     ctxt->vctxt.error = xmlParserValidityError;
     ctxt->vctxt.warning = xmlParserValidityWarning;