Validate preference values set via the CLI. #7346 (#10047)

The CLI set-prefs path (save_pref -> Preferences.save_cli) wrote the raw
value to the configuration database without any type validation and
always reported success, unlike the GUI path which validates via
_Preference.set(). Route save_cli through the same set() validation
(set() now accepts an explicit user_id so it works outside a request
context), and make setup.py set-prefs check the result and report
preferences whose value was invalid.