GRBUFFSZ
err = getgrent_r(&p->grent, p->grbuff, GRBUFFSZ, &ent);
err = fill_grent_from_fd(fd, &p->grent, p->grbuff, GRBUFFSZ);
err = fill_grent_from_fd(fd, &p->grent, p->grbuff, GRBUFFSZ);
char grbuff[GRBUFFSZ]; /* XXX: merge with pwbuff ? */