POSIX-1.2008 per-thread locales, groundwork part 1

POSIX-1.2008 per-thread locales, groundwork part 1

Introduce first cut of struct _thr_locale_t used for the locale_t definition.
Introduce global instance called __global_locale used by default.
Introduce internal inline functions __get_global_locale, __get_locale_r,
__get_current_locale.

Remove usage of global variables in favor of accessor functions pointing to
__global_locale for now.  Include all local headers in locale subdir from
setlocale.h to get single include for internal locale access.

Introduce __CTYPE_PTR macro to replace direct access to __ctype_ptr__
and use throughout in isxxx functions.

Signed-off by: Corinna Vinschen <corinna@vinschen.de>