Add meshlink_set_channel_listen_cb().

Add meshlink_set_channel_listen_cb().

The accept callback is called when the peer has already fully established a
connection. The listen callback is called earlier, when there is no
fully established channel yet. However, the listen callback itself does not
get a channel handle, it can only make a decision based on the peer node
and port number whether to accept the channel, and if so the accept callback
will be called later.