[JdeBP]

I am going with ...

    root # chpst -u 0zero id
    uid=1462(0zero) gid=1481(0zero) groups=1481(0zero)
    root # setuidgid 0zero id
    uid=1462(0zero) gid=1481(0zero) groups=1481(0zero)
    root #

... I would not; I would. Both of these use the getpwnam() library function of course. Their behaviours are ...

    root # chpst -u 0day id
    chpst: fatal: unable to get password/group file entry: file does not exist
    root # setuidgid 0day id
    setuidgid: FATAL: 0day: No such user.
    root #

... to abend when the account name is not found rather than continue on as the superuser without dropping privileges ...

* http://jdebp.eu./Softwares/nosh/guide/setuidgid.html

* http://cr.yp.to/daemontools/setuidgid.html

* http://untroubled.org/daemontools-encore/setuidgid.8.html

... as is documented in their user manuals, resulting in ...

    root # system-control convert-systemd-units ./0day.service
    root # install -d /home/0day
    root # ln -s /etc/service-bundles/services/sysinit-log 0day/log
    root # system-control start 0day 
    root # system-control status 0day
    /var/local/sv/0day: 
            State   : failed since 2017-07-02 11:14:17 +0100; 0s ago
            Main PID: 80083
             Started: exit 0 at 2017-07-02 11:13:35 +0100; 42s ago
                 Ran: exit 111 at 2017-07-02 11:14:17 +0100; 0s ago
            Config  : enabled
    2017-07-02 11:14:17.130870725 setuidgid: FATAL: 0day: No such user.
    2017-07-02 11:14:17.247334398 setuidgid: FATAL: 0day: No such user.
    2017-07-02 11:14:17.363857132 setuidgid: FATAL: 0day: No such user.
    2017-07-02 11:14:17.481175128 setuidgid: FATAL: 0day: No such user.
    2017-07-02 11:14:17.598958702 setuidgid: FATAL: 0day: No such user.
    root #

... the 0day service from the headlined bug report failing to start.