[CLEANUP] get rid of BF_SHUT*_PENDING

BF_SHUTR_PENDING and BF_SHUTW_PENDING were poor ideas because
BF_SHUTR is the pending of BF_SHUTW_DONE and BF_SHUTW is the
pending of BF_SHUTR_DONE. Remove those two useless and confusing
"pending" versions and rename buffer_shut{r,w}_* functions.
diff --git a/src/stream_sock.c b/src/stream_sock.c
index da2004d..3e3e0b9 100644
--- a/src/stream_sock.c
+++ b/src/stream_sock.c
@@ -380,7 +380,7 @@
 			 * we refresh it. A solution would be to merge read+write timeouts into a
 			 * unique one, although that needs some study particularly on full-duplex
 			 * TCP connections. */
-			if (tick_isset(b->rex) && !(b->flags & BF_SHUTR_STATUS))
+			if (tick_isset(b->rex) && !(b->flags & BF_SHUTR))
 				b->rex = b->wex;
 		}
 	}