[JeromeMartinez]

File names on macOS without Unicode, with a locale which is neither ASCII nor UTF-8.

Without Unicode, strings are the bytes of the locale encoding, and the File and Dir functions gave them as they are to the file system, which accepts only valid UTF-8 names on macOS: with e.g. a Latin-1, EUC-JP or GB18030 locale, names typed in a terminal using it could not be opened or created (on Linux and FreeBSD, any bytes are a valid name, so nothing changes there).

• FileName_To_Native() converts the names which are not valid UTF-8 from the locale encoding to UTF-8, and FileName_From_Native() the names of the file system to the locale encoding when it can encode them and the conversion gives back the same name, as with Unicode.
• The File and Dir functions use them also without Unicode (they used them only with Unicode).
• The conversions use iconv of the system, because wchar_t is not Unicode with most of these locales on macOS, with the IANA name of the encoding when iconv does not know the name of the locale (e.g. eucJP). ZenLib is linked with -liconv on macOS (autotools, libzen.pc, libzen-config, CMake); the library is part of macOS, nothing to ship.

To be merged before MediaArea/MediaInfo#1336 and MediaArea/MediaInfoLib#2803, whose tests expect these conversions on macOS without Unicode.