* Reject UID or GID zero unless a documented use case requires it
setup_uid() would happily remap the seafile user to UID/GID 0, which
after the previous non-root fix would mean drop_privileges() drops
straight back into root. main() now rejects --uid/SEAFILE_UID and
--gid/SEAFILE_GID being 0 with exit code 2, before setup_uid() runs.
Verified against the real image via tests/smoke.sh.
* Clarify the GID=0 rejection message
Copilot review: 'group root' isn't a commonly used phrase. Name GID 0
as the root group explicitly instead.
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Co-authored-by: Maxim Shulga <maxim.shulga@alludo.com>