riece-check-channel-commands-are-usable.
(riece-command-invite): Ditto.
(riece-command-kick): Ditto.
(riece-command-change-mode): Ditto.
(riece-command-set-operators): Ditto.
(riece-command-set-speakers): Ditto.
(riece-command-send-message): Ditto.
(riece-command-part): Ditto.
* riece-misc.el (riece-get-users-on-server): If
riece-current-channel is not set, check the default server.
(riece-check-channel-commands-are-usable): New function.