locales: Add #define; change how to override

locales: Add #define; change how to override

This changes the controlling #define for using the POSIX 2008 locale
functions to "USE_POSIX_2008_LOCALE".  The previous controlling name
"USE_THREAD_SAFE_LOCALE" is retained for backward compatibility.

The reason for this change is that we may add thread-safe locale
handling even on platforms that don't have Posix 2008, so the name
USE_THREAD_SAFE_LOCALE would be used for controlling things in that
situation.

In other words, the concepts may become distinct, and so prepare for
that.