core: fix shutdown of --user

core: fix shutdown of --user

Shutting down a user session currently fails with:

Sep 22 22:35:38 david-t2 systemd[640]: Reached target Shutdown.
Sep 22 22:35:38 david-t2 systemd[640]: Starting Exit the Session...
Sep 22 22:35:38 david-t2 systemd[640]: Received SIGRTMIN+24 from PID 659 (kill).
Sep 22 22:35:38 david-t2 systemd[640]: Shutting down.
Sep 22 22:35:38 david-t2 systemd[640]: Not executed by init (PID 1).
Sep 22 22:35:38 david-t2 systemd[640]: Critical error while doing system shutdown: Operation not permitted

This is a regression from:

    commit 287419c119ef961db487a281162ab037eba70c61
    Author: Alban Crequy <alban.crequy@gmail.com>
    Date:   Fri Sep 18 13:37:34 2015 +0200

        containers: systemd exits with non-zero code

Make sure we never ever execute systemd-shutdown from within a
user-manager. Restore the previous behavior by partially reverting given
commit.