locale-util: suppress non-UTF-8 locales when enumerating them

locale-util: suppress non-UTF-8 locales when enumerating them

Let's hide non-UTF-8 locales by default. It's 2019 after all.

Let's add an undocumented env var to reenable listing them though.

This should substantially shorten the list of choices we offer users,
and only show realistic choices.

note that only firstboot and localectl make use of this information, and
both allow configuration of values outside of these lists, hence all
this change does is hide legacy options, but they are still available if
you know what you do, and that's how it should be.