MINOR: receiver: add a receiver-specific flag to indicate the socket is bound

In order to split the receiver from the listener, we'll need to know that
a socket is already bound and ready to receive. We used to do that via
tha LI_O_ASSIGNED state but that's not sufficient anymore since the
receiver might not belong to a listener anymore. The new RX_F_BOUND flag
is used for this.
diff --git a/src/proto_sockpair.c b/src/proto_sockpair.c
index 90300cc..d25ab5a 100644
--- a/src/proto_sockpair.c
+++ b/src/proto_sockpair.c
@@ -106,6 +106,9 @@
 	if (listener->state != LI_ASSIGNED)
 		return ERR_NONE; /* already bound */
 
+	if (listener->rx.flags & RX_F_BOUND)
+		goto bound;
+
 	if (listener->rx.fd == -1) {
 		err |= ERR_FATAL | ERR_ALERT;
 		msg = "sockpair can be only used with inherited FDs";
@@ -122,7 +125,9 @@
 		msg = "cannot make sockpair non-blocking";
 		goto err_return;
 	}
+	listener->rx.flags |= RX_F_BOUND;
 
+ bound:
 	listener->state = LI_LISTEN;
 
 	fd_insert(fd, listener, listener->rx.proto->accept,