Revisiting `passwd`.

Created by: goyox86

• Replaced all unwrap() calls by try().
• Replaced all panic() calls by fail().
• Argument parsing is now all done by ArgParser.
• Used Passwd::parse_file where possible.
• This actually does not change the password, will be the next job.

NOTE: I realised while testing the the password does not get changed and is just printed I left a TODO there and I'll do that in an upcoming PR.