add backlog argument for ConnectionManager::listen_open()

The default backlog in syscall listen(2) is 50. This is too small in our environment. There would be a lot messages
```shell
possible SYN flooding on port 6987. Sending cookies.
```
 in dmesg.
So I make it can be configured.  I would also modify rtorrent to parse an argument in rtorrent.rc to configure it.
Hope it would be useful.