MINOR: listeners: move the LI_O_MWORKER flag to the receiver

This listener flag indicates whether the receiver part of the listener
is specific to the master or to the workers. In practice it's only used
by the master's CLI right now. It's used to know whether or not the FD
must be closed before forking the workers. For this reason it's way more
of a receiver's property than a listener's property, so let's move it
there under the name RX_F_MWORKER. The rest of the code remains
unchanged.
diff --git a/src/cli.c b/src/cli.c
index 90320c2..7f4ea32 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -2601,7 +2601,8 @@
 		l->accept = session_accept_fd;
 		l->default_target = mworker_proxy->default_target;
 		/* don't make the peers subject to global limits and don't close it in the master */
-		l->options |= (LI_O_UNLIMITED|LI_O_MWORKER); /* we are keeping this FD in the master */
+		l->options  |= LI_O_UNLIMITED;
+		l->rx.flags |= RX_F_MWORKER; /* we are keeping this FD in the master */
 		l->nice = -64;  /* we want to boost priority for local stats */
 		global.maxsock++; /* for the listening socket */
 	}