Improve locales (follow-up)

Follow-up to #447.

- On systems with undefined locales (_LOCALE_TEST=BROKEN), explicitly define all LC_ settings as en_US.UTF-8
- On systems with using UTF-8 locales and no undefined locale problems, still explicitly define LC_ settings required for consistency.

See http://unix.stackexchange.com/a/149129/20865 for more background.

LC_CTYPE: although UTF-8 encoding is the most important aspect, and this was already checked for, enforcing en_US.UTF-8 is even safer.
LC_COLLATE=POSIX for consistent results in grep etc.
LC_NUMERIC=POSIX: number formatting: decimal and thousands separator - maybe not directly relevant to BOA scripts, but now that we're touching this, better to choose safe settings.
- On systems with _LOCALE_TEST not broken, still set the same important LC_ settings on the fly, to cover initial installation as well.

- I moved the update-locale command outside the Debian/Ubuntu-specific sections (since it's much longer now).
- Formatted with separate lines for readability.

Happy to hear your comments/questions/suggestions.

By the way, should I generally submit pull requests to master or to the then-current -dev branch?

And, what are your thoughts on code comments? I like to have generous commenting, since it helps me remember why I did something or makes me see more quickly what it does.