server_socket: add method _add_fd()

src/server_socket.h

@@ -43,6 +43,14 @@ server_socket_open(struct server_socket *ss, GError **error_r);
 void
 server_socket_close(struct server_socket *ss);
 
+/**
+ * Add a socket descriptor that is accepting connections.  After this
+ * has been called, don't call server_socket_open(), because the
+ * socket is already open.
+ */
+bool
+server_socket_add_fd(struct server_socket *ss, int fd, GError **error_r);
+
 /**
  * Add a listener on a port on all interfaces.
  *