3160066: Modernize auth in systemd BSI: admin→root and remove NIAuth NSS/PAM integration

[Vairamani-NI]

Summary of Changes

• This PR only includes admin to root changes, NIAuth NSS/PAM removal will be done after discussion with Rich.

Justification

https://dev.azure.com/ni/DevCentral/_workitems/edit/3160066

Testing

• Build locally without errors.
• Tested UID0 is pointing to root.

vairamani@vairamani-ubuntu:~/Documents/mani/nilrt/build$ find tmp-glibc -name passwd | xargs grep ":0:0:"
tmp-glibc/work/x64-nilrt-linux/linux-nilrt-nohz/6.12+git/recipe-sysroot-native/etc/passwd:root::0:0:root:/home/root:/bin/sh
tmp-glibc/work/x64-nilrt-linux/linux-nilrt/6.12+git/recipe-sysroot-native/etc/passwd:root::0:0:root:/home/root:/bin/sh
tmp-glibc/sysroots-components/x86_64/pseudo-native/etc/passwd:root::0:0:root:/home/root:/bin/sh

But, after flashing, admin is still being pulled from NIAuth, so these changes will work only after NIAuth is removed.

login as: root
Pre-authentication banner message from server:
| NI Linux Real-Time (run mode)
|
End of banner message from server
root@172.24.167.45's password:
Last login: Thu Jun 11 13:01:50 2026 from 172.24.160.1
admin@NI-cRIO-903x-VM-5def1b02:~# ls
admin@NI-cRIO-903x-VM-5def1b02:~# ps
  PID TTY          TIME CMD
 3575 pts/0    00:00:00 sh
 3716 pts/0    00:00:00 ps
admin@NI-cRIO-903x-VM-5def1b02:~# id root
uid=0(admin) gid=0(administrators) groups=0(administrators)
admin@NI-cRIO-903x-VM-5def1b02:~# opkg list-installed | grep niauth
libnss-niauth - 26.5.0.150-0+d150
pam-plugin-niauth - 26.5.0.150-0+d150
admin@NI-cRIO-903x-VM-5def1b02:~# opkg list-installed | grep ni-auth
ni-auth - 26.5.0.150-0+d150
ni-auth-networkcontroller - 26.5.0.150-0+d150
ni-auth-webservice - 26.5.0.150-0+d150