intl: Treat C.UTF-8 locale like C locale.

intl: Treat C.UTF-8 locale like C locale.

The wiki page https://sourceware.org/glibc/wiki/Proposals/C.UTF-8
says that "Setting LC_ALL=C.UTF-8 will ignore LANGUAGE just like it
does with LC_ALL=C." This is now implemented in glibc. Implement it
for other platforms as well.

* gettext-runtime/intl/dcigettext.c (DCIGETTEXT, guess_category_value): Treat
C.<encoding> locale like the C locale.
* gettext-runtime/NEWS: Mention it.
* NEWS: Likewise.
* gettext-tools/tests/intl-0: New file, based on gettext-tools/tests/intl-1.
* gettext-tools/tests/Makefile.am (TESTS): Add it.