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/userauth2-client.c

@@ -140,6 +140,7 @@ static const PacketProtocolLayerVtable ssh2_userauth_vtable = {
     .special_cmd = ssh2_userauth_special_cmd,
     .reconfigure = ssh2_userauth_reconfigure,
     .queued_data_size = ssh_ppl_default_queued_data_size,
+    .final_output = ssh_ppl_default_final_output,
     .name = "ssh-userauth",
 };