getent: mem => members

To get the list of group members as a comma-separated list, one should
use $(getent group $GID members) instead of the former $(getent group
$GID mem).

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
getent: Cosmetic cleanups

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>