turn off monitors when logged out

[Tom Horsley]

As far as I know, the login screen runs as a different user
(gdb, for instance, runs as user "gdm").

Since all the settings for power management, screensaver, etc
are per-user settings, to get changes to stick, you need to get
that user to have the right settings (which is non-trivial
since the settings are usually stashed in dconf, but changing
a different user's dconf is tricky, plus you need to discover
the actual names of the relevant settings in the dconf database).