BUG/MINOR: listener: set the listener's fd to -1 after deletion

This is currently harmless, but when stopping a listener, its fd is
closed but not set to -1, so it is not possible to re-open it again.
Currently this has no impact but can have after the abstract sockets
are modified to perform a complete close on soft-reload.

The fix can be backported to 1.5 and may even apply to 1.4 (protocols.c).
(cherry picked from commit 39447b6a5799a160eae452db920fd0735a78638b)

src/listener.c

@@ -236,6 +236,7 @@ int unbind_listener(struct listener *listener)
 
 	if (listener->state >= LI_PAUSED) {
 		fd_delete(listener->fd);
+		listener->fd = -1;
 		listener->state = LI_ASSIGNED;
 	}
 	return ERR_NONE;