Comment 9 for bug 1473800

Worked for me on trusty.

I had trouble with sudo apt-get install systemd-services/trusty-proposed -- it wanted to uninstall too many things. Probably it would have worked fine with sudo apt-get install systemd-services/trusty-proposed libpamd-systemd/trusty-proposed. Oops.

I instead asked apt to upgrade all the binary packages built from the systemd source package:
sudo apt-get install -t trusty-proposed gir1.2-gudev-1.0 libgudev-1.0-0 libpam-systemd libsystemd-daemon0 libsystemd-journal0 libsystemd-login0 libudev1 libudev1 systemd-services udev

This restarted systemd-logind as expected (and blanked my screen momentarily).

I asked apt to re-install all those packages:
sudo apt-get install -t trusty-proposed --reinstall gir1.2-gudev-1.0 libgudev-1.0-0 libpam-systemd libsystemd-daemon0 libsystemd-journal0 libsystemd-login0 libudev1 libudev1 systemd-services udev

This time systemd-logind did not restart and my screen did not blank.

Thanks Martin!