[MINOR] re-arrange buffer flags and rename some of them

The buffer flags became a big bazaar. Re-arrange them
so that their names are more explicit and so that they
are more easily readable in hex form. Some aggregates
have also been adjusted.
diff --git a/src/senddata.c b/src/senddata.c
index dea2c96..5d4b8a6 100644
--- a/src/senddata.c
+++ b/src/senddata.c
@@ -60,7 +60,7 @@
 		buffer_write(s->rep, msg->str, msg->len);
 
 	s->rep->wex = tick_add_ifset(now_ms, s->rep->wto);
-	s->rep->flags |= BF_MAY_FORWARD;
+	buffer_write_ena(s->rep);
 }