sshshare: notify cl when last downstream goes away.

The check_termination function in ssh2connection is supposed to be
called whenever it's possible that we've run out of (a) channels, and
(b) sharing downstreams. I've been calling it on every channel close,
but apparently completely forgot to add a callback from sshshare.c
that also arranges to call it when we run out of downstreams.

ssh1connection.c

@@ -119,6 +119,7 @@ static const struct ConnectionLayerVtable ssh1_connlayer_vtable = {
     NULL /* alloc_sharing_channel */,
     NULL /* delete_sharing_channel */,
     NULL /* sharing_queue_global_request */,
+    NULL /* sharing_no_more_downstreams */,
     ssh1_agent_forwarding_permitted,
     ssh1_terminal_size,
     ssh1_stdout_unthrottle,