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).

(cherry picked from commit d6e6919f69)

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",
 };