su_to_zulip: Fix detection of zulip user ID.
su_to_zulip: Fix detection of zulip user ID.

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.

shubhamdhama/zulip,punchagan/zulip,kou/zulip,tommyip/zulip,brainwane/zulip,eeshangarg/zulip,showell/zulip,andersk/zulip,shubhamdhama/zulip,kou/zulip,synicalsyntax/zulip,timabbott/zulip,synicalsyntax/zulip,kou/zulip,shubhamdhama/zulip,punchagan/zulip,andersk/zulip,kou/zulip,kou/zulip,timabbott/zulip,shubhamdhama/zulip,hackerkid/zulip,synicalsyntax/zulip,rht/zulip,rishig/zulip,tommyip/zulip,tommyip/zulip,shubhamdhama/zulip,showell/zulip,timabbott/zulip,hackerkid/zulip,brainwane/zulip,eeshangarg/zulip,hackerkid/zulip,hackerkid/zulip,eeshangarg/zulip,rishig/zulip,kou/zulip,zulip/zulip,zulip/zulip,punchagan/zulip,showell/zulip,rht/zulip,andersk/zulip,rishig/zulip,hackerkid/zulip,brainwane/zulip,showell/zulip,kou/zulip,eeshangarg/zulip,tommyip/zulip,rht/zulip,shubhamdhama/zulip,shubhamdhama/zulip,rht/zulip,andersk/zulip,zulip/zulip,timabbott/zulip,timabbott/zulip,synicalsyntax/zulip,punchagan/zulip,zulip/zulip,eeshangarg/zulip,tommyip/zulip,rishig/zulip,brainwane/zulip,tommyip/zulip,brainwane/zulip,synicalsyntax/zulip,rishig/zulip,brainwane/zulip,rishig/zulip,showell/zulip,eeshangarg/zulip,brainwane/zulip,rht/zulip,showell/zulip,punchagan/zulip,rishig/zulip,tommyip/zulip,showell/zulip,zulip/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,andersk/zulip,punchagan/zulip,timabbott/zulip,andersk/zulip,hackerkid/zulip,synicalsyntax/zulip,synicalsyntax/zulip,rht/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,timabbott/zulip,andersk/zulip