This module can be used to read and manipulate entries in Unix system
password files (/etc/passwd, /etc/group, /etc/group, /etc/gshadow; but can
also be told to search in custom location, for testing purposes).

This module uses a procedural (non-OO) interface. Each function in this
module open and read the passwd files once. Read-only functions like
`list_users()` and `get_max_gid()` open in read-only mode. Functions that
might write to the files like `add_user()` or `delete_group()` first lock
`passwd.lock` file, open in read+write mode and also read the files in the
first pass, then seek to the beginning and write back the files.

No caching is done so you should do your own if you need to.