shell: "Logout" -> "Terminate sessions" on account page.

"Logout" is used for the currently logged in user.

Fixes #76
shell: Allow password changing for root.

By optionally calling passwd directly instead of going via
AccountsService.
shell: Explicitly allow "root" to do everything.

Previously, "root" was implicitly (and accidentally) handled as part
of the "When in doubt, just go ahead and let it fail later" fallback
path.
shell: Replace "Account Settings" with "Change Password" for root.

Since we can't show the account page for root.