We were leaking various references, leading to failures to populate
the list upon reloading, or to segfaults. With this commit, all
created user objects, as well as the user-manager object are
finalized when the panel is unloaded.
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
