parser: Improve error handling

Introduce xmlCtxtSetErrorHandler allowing to set a structured error for
a parser context. There already was the "serror" SAX handler but this
always receives the parser context as argument.

Start to use xmlRaiseMemoryError.

Remove useless arguments from memory error functions. Rename
xmlErrMemory to xmlCtxtErrMemory.

Remove a few calls to xmlGenericError.

Remove support for runtime entity debugging.

include/libxml/parserInternals.h

@@ -314,8 +314,7 @@ XMLPUBFUN xmlParserCtxtPtr
 						 const xmlChar *ID,
 						 const xmlChar *base);
 XMLPUBFUN void
-			xmlErrMemory		(xmlParserCtxtPtr ctxt,
-						 const char *extra);
+			xmlCtxtErrMemory	(xmlParserCtxtPtr ctxt);
 XMLPUBFUN int
 			xmlSwitchEncoding	(xmlParserCtxtPtr ctxt,
 						 xmlCharEncoding enc);