[MAJOR] struct chunk rework

Add size to struct chunk and simplify the code as there is
no longer required to pass sizeof in chunk_printf().
diff --git a/src/haproxy.c b/src/haproxy.c
index e448f4f..d426e67 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -704,7 +704,7 @@
 		free(p->monitor_uri);
 
 		for (i = 0; i < HTTP_ERR_SIZE; i++)
-			free(p->errmsg[i].str);
+			chunk_destroy(&p->errmsg[i]);
 
 		for (i = 0; i < p->nb_reqadd; i++)
 			free(p->req_add[i]);