Update gnome-session commands for GNOME 3

gnome-session-quit has no --reboot option, and --power-off doesn't
respect --no-prompt, so just make the appropriate dbus calls directly.
Fix root's reboot command
Don't start a demo client or lock screen on master

If a demo server is running, probably the management interface is as
well. Avoid starting a demo or locking the screen in that case.