intl: Treat C.UTF-8 locale like C locale, part 2 (BZ# 16621)

intl: Treat C.UTF-8 locale like C locale, part 2 (BZ# 16621)

The previous commit was incomplete: gettext() still returns a translation
if the file /usr/share/locale/C/LC_MESSAGES/<domain>.mo exists. This patch
prohibits the translation also in this case.

* gettext-runtime/intl/dcigettext.c (DCIGETTEXT): Treat C.<encoding> locale
like the C locale.

Reviewed-by: Florian Weimer <fweimer@redhat.com>