[CFT] pw(8) using pw/gr API from libutil

Hi all,

I have been modifying pw(8) it order for it to user the pw_* and gr_* function
from libutil, to: avoid code duplication have a single API to manipulate
user/group things.

I have been using this version for a while now and it works quite well to me,
I'm just missing testing in real NIS environment.

The patch can be find here:
http://people.freebsd.org/~bapt/pw_use_pw_gr_utils.diff

regards,
Bapt