Override any locale environment variable

In one case reported on IRC, having LC_CTYPE=da_DK.UTF8 caused mpstat to report "Middel" instead of "Average".

We had LANG=POSIX but that was obviously not enough. Using LC_ALL overrides all local variables and gets us the desired result.

http://pubs.opengroup.org/onlinepubs/007908799/xbd/envvar.html