patched to redirrect all "out of context" error messages to

a reconfigurable routine. The changes are:
* xmlerror.h : added the export of an error context type (void *)
  an error handler type xmlGenericErrorFunc there is an interface
  xmlSetGenericErrorFunc(void *ctx, xmlGenericErrorFunc handler);
  to reset the error handling routine and its argument
  (by default it's equivalent to respectively fprintf and stderr.
* all the c files: all wild accesses to stderr or stdout within
  the library have been replaced to calls to the handler.
Daniel

testSAX.c

@@ -617,7 +617,8 @@ void parseAndPrintFile(char *filename) {
 	    }
 	    fclose(f);
 	} else {
-	    fprintf(stderr, "Cannot read file %s\n", filename);
+	    xmlGenericError(xmlGenericErrorContext,
+		    "Cannot read file %s\n", filename);
 	}
 	/*
 	 * Debug callback