Win32: Unicode file name support (dirent)
Changes opendir/readdir to use Windows Unicode APIs and convert between
UTF-8/UTF-16.
Removes parameter checks that are already covered by xutftowcs_path. This
changes detection of ENAMETOOLONG from MAX_PATH - 2 to MAX_PATH (matching
is_dir_empty in mingw.c). If name + "/*" or the resulting absolute path is
too long, FindFirstFile fails and errno is set through err_win_to_posix.
Increases the size of dirent.d_name to accommodate the full
WIN32_FIND_DATA.cFileName converted to UTF-8 (UTF-16 to UTF-8 conversion
may grow by factor three in the worst case).
Signed-off-by: Karsten Blees <blees@dcon.de>