- if pw is NULL and oldpw is not NULL then the oldpw is deleted
- if pw->pw_name != oldpw->pw_name but pw->pw_uid == oldpw->pw_uid
then it renames the user
add new gr_* functions so now gr_util API is similar to pw_util API,
this allow to manipulate groups in a safe way.
Reviewed by: des
Approved by: des
MFC after: 1 month