i18n: Use Gnulib's `nl_langinfo' module.

i18n: Use Gnulib's `nl_langinfo' module.

* configure.ac: Remove checks for <langinfo.h> and <nl_types.h>, and
  `nl_langinfo'.

* libguile/i18n.c: Remove #ifdefs for HAVE_LANGINFO_H, HAVE_NL_TYPES_H,
  HAVE_NL_ITEM, and HAVE_LANGINFO_CODESET.
  (SCM_VALIDATE_OPTIONAL_LOCALE_COPY): Use `SCM_UNBNDP'.

* m4/gnulib-cache.m4 (gl_MODULES): Add `nl_langinfo'.

* module/ice-9/i18n.scm (define-vector-langinfo-mapping): Remove
  `defaults' parameter; assume (provided? 'nl-langinfo) is always true.
  Update users accordingly.
  (define-simple-langinfo-mapping): Likewise.
  (define-monetary-langinfo-mapping): Likewise, but do not assume
  LOCAL-ITEM and INTL-ITEM are always defined.