i18n: Always use locale-dependent string collation.

i18n: Always use locale-dependent string collation.

* libguile/i18n.c (compare_u32_strings, compare_u32_strings_ci): Always
  use locale-dependent string collation.

* test-suite/tests/i18n.test: Recoded in UTF-8.
  (%french-utf8-locale-name): New.
  (under-locale-or-unresolved): New.  Don't catch errors on GNU systems.
  (under-french-locale-or-unresolved): Use it.
  (under-french-utf8-locale-or-unresolved): New.
  ("text collation (French)")["string-locale-ci=? (2 args, wide
  strings)", "string-locale-ci=? (3 args, wide strings)",
  "string-locale-ci<>? (wide strings)", "string-locale-ci<>? (wide and
  narrow strings)", "char-locale-ci<>? (wide)"]: New tests.