util.c:is_locale_utf8(): check, if "C" was set on purpose

If you have a ASCII only terminal, there is no way to set the charmap to
ANSI_X3.4-1968, other than using LC_CTYPE=C.

We don't want to assume a UTF-8 capable terminal in this case and only
do so, if LANG, LC_ALL and LC_CTYPE are unset.