Packet protocol layers: new 'final_output' method.

This is called just before closing the connection, and gives every PPL
one last chance to output anything to the user that it might have
buffered.

No functional change: all implementations so far are trivial, except
that the transport layer passes the call on to its higher
layer (because otherwise nothing would do so).

ssh/connection2.c

@@ -27,6 +27,7 @@ static const PacketProtocolLayerVtable ssh2_connection_vtable = {
     .special_cmd = ssh2_connection_special_cmd,
     .reconfigure = ssh2_connection_reconfigure,
     .queued_data_size = ssh_ppl_default_queued_data_size,
+    .final_output = ssh_ppl_default_final_output,
     .name = "ssh-connection",
 };