newgrp: add support for /etc/gshadow

newgrp: add support for /etc/gshadow

The original newgrp command doesn't expect group pasword in /etc/gshadow
although almost all distributions use this file (and the gpasswd command).

The newgrp from util-linux is deprecated and better is use shadow-utils only.
Unfortunately, shadow-utils are broken too (see RH version where is bugfix).
In this case it's better fix util-linux version at least...

Signed-off-by: Karel Zak <kzak@redhat.com>