RE making UnixMain public, I'd rather the core runtime require a known encoding, rather than trying to detect it. We should move the call into the detection logic into Programs/python.c so that embedders have to opt-in to detection (many embedding scenarios will prefer to do their own encoding).

Provided it's Unicode, I don't care whether it's a char or wchar API. Windows can always reliably convert to UTF-8, so if Linux needs some extra help here by using char then that's fine.