MINOR: listeners: move fd_stop_recv() to the receiver's socket code

fd_stop_recv() has nothing to do in the generic listener code, it's per
protocol as some don't need it. For instance with abns@ it could even
lead to fd_stop_recv(-1). And later with QUIC we don't want to touch
the fd at all! It used to be that since commit f2cb169487 delegating
fd manipulation to their respective threads it wasn't possible to call
it down there but it's not the case anymore, so let's perform the action
in the protocol-specific code.
diff --git a/src/proto_uxst.c b/src/proto_uxst.c
index bafe97d..c477d19 100644
--- a/src/proto_uxst.c
+++ b/src/proto_uxst.c
@@ -151,7 +151,8 @@
 /* Pause a listener. Returns < 0 in case of failure, 0 if the listener
  * was totally stopped, or > 0 if correctly paused. Nothing is done for
  * plain unix sockets since currently it's the new process which handles
- * the renaming. Abstract sockets are completely unbound.
+ * the renaming. Abstract sockets are completely unbound and closed so
+ * there's no need to stop the poller.
  */
 static int uxst_pause_listener(struct listener *l)
 {