unity's lock screen doesn't respect gsettings

[jdreed]

It is not possible to disable "Switch User" on unity's new lock screen. The various tutorials indicating it can be done with ccsm pre-date the Trusty final release. I'm sure upstream will tell us we're wrong for wanting this feature, so we should consider workarounds. A couple come to mind:

• Configure the lightdm greeter to display a "Go away" message if it is started while a session is active (dm-tool will give us this info, and probably ConsoleKit? too?)
• Do something hilarious with /usr/share/upstart/sessions/unity-panel-service-lockscreen.conf
• Restore xscreensaver (you know you want to!)
• Something else