Implement usermod, groupmod, userdel, groupdel; Fix passwd and useradd
Here I've implemented the remainder of the core user utilities present on most unix systems. They aren't particularly pretty, but they do work for the most part. Note that for some reason attempting to move the home directory of a user via usermod -m does not work. Also removing user's that have been deleted from groups is not yet implemented, nor is updating /etc/passwd when a group id is changed. Note that redox_users as used by Cargo.lock as of this commit is broken, and may not work properly. If redox-os/users#11 is merged, cargo update should fix.
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