systemctl: keep ignoring sessions on shutdown as root

systemctl: keep ignoring sessions on shutdown as root

The change was supposed to be about respecting inhibitors, but
it was extended to also error out when there are active user
sessions, which was not intentional. Previously systemctl skipped
all checks if the caller was root or root-equivalent. Restore the
previous behaviour and again avoid blocking systemctl reboot by root
if there are active sessions, as long as there are no active
inhibitors.

Fixes https://github.com/systemd/systemd/issues/34086

Follow-up for 804874d26ac73e0af07c4c5d7165c95372f03f6d