add fallback for gid and uid lookup

[ktmeaton]

This pull request adds fallback values for group and user on systems where the lookup fails. I'm not sure if this is actually a good idea, but I'd guess that ls does something similar.

[mattheww95]

I have been going through the coreutils code and I am still not quite sure what exactly is responsible for getting the gid when it is not in /etc/groups but I think in this case a solution is better than nothing even if there are edge cases.

Thank you so much for fixing this, It has given me some additional reading as well. I did not know you could write literal c strings in rust like you did.

Thanks so much once again!