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.