devel/man-to-mdwn.pl: portable locale environment variable setting

devel/man-to-mdwn.pl: portable locale environment variable setting

Setting locale environment variables (LC_* and LANG) to e.g.
en_US.utf8 works fine on Linux, and that is what locale -a
returns (in Linux). However this does not work e.g. in some *BSD
systems.
In these systems, en_US.UTF-8 works. This also works in Linux
systems (which may look like a surprising thing on the first sight(*)).
But that *UTF-8 format seems to be widely used in the Linux system:
Grep it through the files in /etc/, for example.

Easy way to test: Run the following command lines. First should
complain about setting locale failed, and second should not.

$ LC_ALL=en_US.UTF-1 perl -e ''
$ LC_ALL=en_US.UTF-8 perl -e ''

(*) and who knows what the "standard" is...