Rewrite parsing subcommands arguments of pw(8)

Rewrite parsing subcommands arguments of pw(8)

Now each subcommands checks its arguments in a dedicated functions.

This helps improving input validation, code readability/maintainability
While here:
- Add a -y option to pw userdel/usermod so it can maintain NIS servers if
  nispasswd is not defined in pw.conf(5)
- Allow pw -r <rootdir> to remove directory with userdel -r
- Fix bug when renaming a user which was not renaming the user name it groups
  it is a member of.
- Only parse pw.conf(5) when needed.