debian/atopacct.init: fix return code for start if already running:

systemd does not like 1 as return code, thus returning 0 for now, as
otherwise the package fails to install when atopacctd is already running.

Sep 15 16:33:25 mango systemd[1]: atopacct.service: control process exited, code=exited status=1
Sep 15 16:33:25 mango systemd[1]: Failed to start LSB: Process accounting control.
-- Subject: Unit atopacct.service has failed
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
--
-- Unit atopacct.service has failed.
--
-- The result is failed.