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

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

Fixes #76
Reviewed-by: Stef Walter <stefw@redhat.com>
shell: Allow password changing for root.

By optionally calling passwd directly instead of going via
AccountsService.

Reviewed-by: Stef Walter <stefw@redhat.com>
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.

Reviewed-by: Stef Walter <stefw@redhat.com>
shell: Replace "Account Settings" with "Change Password" for root.

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

Closes #805
Reviewed-by: Stef Walter <stefw@redhat.com>