0029081: With Mingw-w64 Unicode Paths Do Not Work

0029081: With Mingw-w64 Unicode Paths Do Not Work

OSD_OpenStream() now uses __gnu_cxx::stdio_filebuf extension for opening UNICODE files on MinGW when using C++ file streams.
Variant accepting filebuf returns bool (true if succeeded and false otherwise).

Checks of ofstream to be opened made via calls to low-level ofstream::rdbuf() are replaced by calls to ofstream::is_open(); state of the stream is also checked (to be good).
Unicode name used for test file in test bugs fclasses bug22125 is described (for possibility to check it).