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.
shell: Replace "Account Settings" with "Change Password" for root.

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