Reject UID or GID zero unless a documented use case requires it (#32)

* 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.

---------

Co-authored-by: Maxim Shulga <maxim.shulga@alludo.com>

start.py

@@ -131,6 +131,12 @@ def main():
         parser.error("username is not specified")
     if not args.libraries:
         parser.error("library is not specified")
+    if args.uid == 0:
+        parser.error("uid must not be 0; running the daemon as root defeats "
+                      "the point of --uid/SEAFILE_UID")
+    if args.gid == 0:
+        parser.error("gid must not be 0 (the root group); running the daemon "
+                      "with it defeats the point of --gid/SEAFILE_GID")
 
     try:
         password = resolve_secret("PASSWORD", args.password, args.password_file)