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

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 patch implements it.

* intl/dcigettext.c (guess_category_value): Treat C.<encoding> locale
like the C locale.

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