New helper: ensure_locales()
Speed up ensure_locales()

So locale-gen --no-purge doesn't do what I think it would do.

I've no idea why locale-gen is regenerating everything almost all the
time.

I've no idea how locale-gen works actually.  It doesn't seem to match
the behavior documented in the manual page.
install_packages(): fix default of missing_only to False

I meant to do that before and documented the change, but forgot to do
the change.