Improve fallback during locale initialization

Improve fallback during locale initialization

If Perl encounters a problem during startup trying to initialize the
locales from the environment it has immediately reverted to the "C"
locale.

This commit generalizes that so it tries each of the applicable
environment variables in order of priority until it works, or it gives
up and uses the "C" locale.  For example, if LC_ALL is set to something
that is invalid, but LANG is valid, LANG will be used.  This was
motivated by trying to get the Windows system default locale used in
preference to "C" if all else fails.