Respect locale environment variables on darwin. In python 2.4+ on darwin, locale.getpreferredencoding() returns mac-roman regardless of what LC_CTYPE, LANG etc are set to. This can produce hard-to-notice conversion errors if input text is not in mac-roman. So this patch overrides it with setlocale/getlocale if the environment has been customized, on the assumption that the user has done so deliberately.