Allow backlog to be updated

This commit adds support to the sockets and netconn layer to update the
backlog by calling listen when the netconn is already in the listen state.
When backlog is not enabled, the call returns successfully

This commit also introduces a macro for setting the backlog value that
prevents a 0 sized (invalid) backlog

src/api/api_msg.c

@@ -1308,6 +1308,12 @@ lwip_netconn_do_listen(struct api_msg_msg *msg)
               }
             }
           }
+        } else if (msg->conn->state == NETCONN_LISTEN) {
+          /* already listening, allow updating of the backlog */
+          msg->err = ERR_OK;
+#if TCP_LISTEN_BACKLOG
+          tcp_backlog_set(msg->conn->pcb.tcp, msg->msg.lb.backlog);
+#endif /* TCP_LISTEN_BACKLOG */
         }
       } else {
         msg->err = ERR_ARG;