path locale-related functions are not thread-safe

[Andrey Semashev]

Boost.Filesystem v.3 keeps the locale used by path objects in a function-local static object (see path_locale function in path.cpp). If multiple threads use path methods that involve locale (such as string(), wstring(), converting constructors or assignment from strings), the code may use a not yet constructed locale object or leak the locale. The locale object must be protected with call_once or similar technique.

[Beman Dawes]

(In [76303]) Fix #4889, #6320, Locale codecvt_facet not thread safe on Windows. Move Windows, Mac OS X, locale and codecvt facet back to namespace scope. POSIX except OS X uses local static initialization (IE lazy) to ensure exceptions are catchable if environmental variables are misconfigured and to avoid use of locale("") if not actually used.