check differences between chpasswd and chgpasswd

check IS_ISLNK in lib/defines.h

ALL:
- move base passwd/shadow/group/gshadow operation to module for allow write
  different backend modules for db, NIS, LDAP and others. Default backend it
  will be goot if will be chosen depending on /etc/nsswitch.conf and allow
  override this by -r <repository> options (where the <repository> can be
  file, db, nis nisplus, ldap .. like on /etc/nsswitch.conf in service column).
  passwd have old piece of code with handling -r option and it will be good
  finish this and propagate on other shadow tools for allow operate on other
  user databases by well known tools.
- Add/check support for split group
  Maybe that feature can be supported in gr_open/gr_close, with a variable
  in login.defs.

- useradd:
  - add handle create user mail spool in maildir format.

- add handle -n switch in groups and id command for allow query is
  group/user with specified id/gid exist - this will be very usable
  on automation in packages for query/check is group/user exist in system
  or not,

- groupmems: 
  - need some work on add PAM and i18n support.

- userdel:
  - add backup option for the removal of user resources,
  - add lookop and remove per user group.
  - user_busy: check that the user is not running any processes.

- passwd:
  - check combination of options (e.g. -u/-l)

- newgrp: check the USE_PAM section.

- groupmems: check reason for isgroup

- chgpasswd: uses chpasswd's pam

- groupconv: i = 100; no a nice initial value, use login.defs

Documentation:

- document the -s option of {pw,grp}ck in the pt_BR, and ja man pages

- document -o option in ja man page,