The locale machinery on OSX is flaky. The question is what people really
pass for command line arguments. It would be useful to find out what
happens in these two cases:

1. Somebody runs "a.py ภาษาไทย" in a Terminal.app window. Most likely,
the terminal encoding is applied, which we should assume to be UTF-8
(although it might be different on some systems).

2. Somebody creates a file japanese_コンテンツ in the finder, then uses
shell completion to pass this to a Python script. Here I expect that
UTF-8 is used even if the terminal's encoding is not UTF-8.

I don't know whether it's possible to launch Python scripts from Finder,
for given files, if so, it would also be interesting to find out what
encoding will be used there.

Without actual testing, I would assume that command line arguments are
typically encoded in UTF-8 on OSX. We should use that for argument
processing, regardless of mbstowcs.