Set locale to en_US.UTF-8 only

Too many remote servers don't have en_GB.UTF-8 which leads to annoying
error messages. Setting locale to en_US.UTF-8 as workaround.

man locale says LANG is a substitute for LC_* and LC_ALL overrides LC_*,
I'm guessing that setting LC_ALL should be enough. As for LC_COLLATE I
don't even remember why I set it to C, so let's just use en_US.UTF-8 for
that too until there is a reason to change.