Allow custom files to be opened and allow sorting by timestamp.

While implementing a tool to import lastlog entries into utmpx, I
noticed lastlogin doesn't allow custom database files to be opened. Add
a -f switch to support this. Also, add -r and -t similar to ls(1),
ruptime(1), etc. where you can sort entries by timestamp and reverse
them. This allows you to spot active/idle users more easily.