Port to redox_users break-api

All the utilities use the new API I defined in mu recent PR to redox_users. su's behavior was also fixed. Also fixed passwd so that it actually persists changes to the filesystem.

I also updated Cargo.toml (removing unused deps). This may require another commit after my first PR is merged in order to appropriately update Cargo.lock and make sure everything builds.

This PR is very dependent on https://github.com/redox-os/users/pull/8, do not merge unless that has also been merged! I would also recommend waiting until I can double check to make sure this builds with upstream having been updated (my git process was not what it should have been...)