Apparently, while upgrade-zulip-from-git always ensures that zulip
deployment directories are owned by the Zulip user, unpack-zulip (aka
the tarball code path) has them owned by root.
The user ID detection logic in su_to_zulip's helper get_zulip_uid was
intended to support both development environments (where the user ID
might vary) and production environments. For development
environments, the existing code is fine, but given this unpack-zulip
permissions issue, we need to have code to fallback to 'zulip' if the
detection logic detects the "zulip" user has having UID 0.
870cd00f5f