localed: Run locale-gen if available to generate missing locale

localed: Run locale-gen if available to generate missing locale

This change improves integration with distributions using locale-gen to
generate missing locale on-demand, like Debian-based distributions
(Debian/Ubuntu/PureOS/Tanglu/...) and Arch Linux.
We only ever enable new locales for generation, and never disable them.
Furthermore, we only generate UTF-8 locale.

This feature is only used if explicitly enabled at compile-time, and
will also be inert at runtime if the locale-gen binary is missing.