Bug#1027999: xscreensaver: xscreensaver-systemd does not start: "inhibit sleep failed: Permission denied"

[Jamie Zawinski]

What does polkitd do, and why does xscreensaver-systemd fail without it?

I gather it has something to do with "org.freedesktop.PolicyKit1" but I can't 
tell what that's for either, and xscreensaver-systemd does not explicitly use 
it.