Expect an exit code of 15 as success as atop daemon does not reraise the signal...

... as systemd seems to expect it to:

atop does:

various.c: cleanstop(exitcode)
{
        acctswoff();
        netatop_signoff();
        (vis.show_end)();
        exit(exitcode);
}

atop.c:
        /*
        ** catch signals for proper close-down
        */
        signal(SIGHUP,  cleanstop);
        signal(SIGTERM, cleanstop);

systemd expects:

signal(SIGTERM, SIG_DFL, SIGTERM)