nixos: reserve some uids/gids

I have some NixOS modules that I keep out of tree, and having UIDs/GIDs
reserved is quite helpful.

Signed-off-by: Austin Seipp <aseipp@pobox.com>
Merge branch 'reserve-ids' of git://github.com/thoughtpolice/nixpkgs

nixos: reserve some uids/gids (#2109)