(TopLevel): Require `gnus-group' at the compile time.

(TopLevel): Require `gnus-group' at the compile time.
(gnus-offline-set-online-sendmail-function): defsubst.
(gnus-offline-set-offline-sendmail-function): Ditto.
(gnus-offline-set-offline-post-news-function): Ditto.
(gnus-offline-set-online-post-news-function): Ditto.
(gnus-offline-disable-fetch-mail): Ditto.
(gnus-offline-enable-fetch-mail): Ditto.
(gnus-offline-setup): Fix typo.
(gnus-offline-gnus-get-new-news): Abolish.
(gnus-offline-toggle-plugged): Ditto.
(gnus-offline-agent-expire): Ditto.
(gnus-group-get-new-news): New advice which does things
`gnus-offline-gnus-get-new-news' was doing.
(gnus-agent-toggle-plugged): New advice which does thing
`gnus-offline-toggle-plugged' was doing.
(gnus-agent-expire): New advice which does things
`gnus-offline-agent-expire' was doing.
(gnus-offline-define-menu-and-key): No longer substitute key
definitions on `gnus-group-mode-map'. No longer swap commands for
a toolbar button.
(gnus-offline-after-get-new-news): Do jobs only when
`gnus-plugged' is t.