WIP checkpoint for progress callbacks in the API.

At this moment I've only implemented the part for unpacking, other parts
will also use progress callbacks.

--HG--
branch : progress_callback

lib/initend.c

@@ -76,3 +76,14 @@ xbps_dbg_printf_append(const char *fmt, ...)
 	vfprintf(stderr, fmt, ap);
 	va_end(ap);
 }
+
+void
+xbps_error_printf(const char *fmt, ...)
+{
+	va_list ap;
+
+	va_start(ap, fmt);
+	fprintf(stderr, "ERROR: ");
+	vfprintf(stderr, fmt, ap);
+	va_end(ap);
+}