MINOR: connections: Add a flag to know if we're in the safe or idle list.

Add flags to connections, CO_FL_SAFE_LIST and CO_FL_IDLE_LIST, to let one
know we are in the safe list, or the idle list.
diff --git a/include/proto/server.h b/include/proto/server.h
index ca794ab..7ba9c83 100644
--- a/include/proto/server.h
+++ b/include/proto/server.h
@@ -262,6 +262,8 @@
 			return 0;
 		}
 		MT_LIST_DEL(&conn->list);
+		conn->flags = (conn->flags &~ CO_FL_LIST_MASK) |
+		              (is_safe ? CO_FL_SAFE_LIST : CO_FL_IDLE_LIST);
 		MT_LIST_ADDQ(is_safe ? &srv->safe_conns[tid] : &srv->idle_conns[tid],
 		             (struct mt_list *)&conn->list);
 		srv->curr_idle_thr[tid]++;