Language-specific case-conversion doesn't honor locale

Language-specific case-conversion doesn't honor locale

Libunistring uses a function uc_locale_language to extract the
current language from the locale information.  It does this by calling
setlocale.  This makes incompatible with Guile functions that use the
locale_t thread-specific locale API, because the values returned by the
call to setlocale ignore the locale set by uselocale.

As a workaround, this patch extracts the language from the locale_t
structure's __names field.

A more complete solution is needed.  Perhaps that solution would test
that the __names field exists in the configure step and revert to
!USE_GNU_LOCALE_API in that case.

* libguile/i18n.c (locale_language): new function that performs the
  same job as uc_locale_language but is compatible with uselocale
  (u32_locale_casecoll, u32_locale_tocase): replace uc_locale_language
  with locale_language