When used with accountsservice >= 0.6.9, this fixes the problem where,
when you click the unlock button on the user panel, the root user pops
up in the user list, unexpectedly.
When used with 0.6.8 it silently does nothing, since the system-account
property is not available.
https://bugzilla.gnome.org/show_bug.cgi?id=647487