Fix comments.

Fix comments.
(TopLevel): Delete the code for emulating custom. Do not inhibit
byte-compile-warnings, but hide useless ones.
(gnus-offline-dialup-program-arguments): defvar instead of
defcustom.
(gnus-offline-hangup-program-arguments): Ditto.
(gnus-offline-interval-time): Ditto.
(gnus-offline-dialup-program, gnus-offline-hangup-program,
gnus-offline-drafts-queue-type, gnus-offline-MTA-type): defvar.
(gnus-offline-disable-fetch-mail): Remove pop3-fma dependent codes.
Set `mail-sources' instead of `nnmail-spool-file'.
(gnus-offline-enable-fetch-mail): Ditto.
(gnus-offline-toggle-movemail-program): Abolish.
(gnus-offline-define-menu-and-key): Modify according to it.
(gnus-offline-define-menu-on-miee): Ditto.
(gnus-offline-define-menu-on-agent): Ditto.
(gnus-offline-message-add-header): Bind temporary variables.
(gnus-offline-add-custom-header): Ditto.
(gnus-offline-restore-mail-group-level): Ditto.