Timout, Iters, Bugfix

Here I did two major things:

AllGroups and AllUsers now have iter() and iter_mut() to iterate over them. Not really sure if this is the correct way to implement iteration here, so advice would be really nice if it's not.

Implemented a timeout when passwords are incorrect. All we do is sleep if the password is not verified.

Also fixed a little bug with Group::to_string(), and implemented a shared lock when reading from /etc/passwd and /etc/group (forgot to put that in the commit message)