Avoid uid/gid 0 metadata tests when ids don't exist

Avoid uid/gid 0 metadata tests when ids don't exist

Cygwin may not have a 0 uid/gid, so skip the relevant tests whenever
it doesn't.

We have to explicitly exclude the gid 0 from other_group, because
/etc/group may define a group named root with the same id as
Administrators. We cannot use "root" instead of 0, because root may
not be defined.

Cygwin users: If you want to define the root group, add this line
at the begining of /etc/group:
  root:S-1-5-32-544:0:

Signed-off-by: Patrick Rouleau <prouleau72@gmail.com>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>