[Toybox] internationalization with sprintf("%.123s")

[Rob Landley]

So this bug:

https://sourceware.org/bugzilla/show_bug.cgi?id=6530

Brought to my attention the fact that %s length limiters are
_characters_, not bytes. The default locale is C so it's equivalent up
until you call setlocale(), and then suddenly there's heap and stack
overflow possibilities all over the place.

To fix this I added a TOYFLAG_LOCALE that you have to set in a command
that wants setlocale() called on the way in. It no longer happens by
default when you enable CFG_I18N.

Just FYI: who would have thought that complicating the system and
changing the behavior of all sorts of base library functions in subtle
ways would open security holes?

Rob