Working on "Use std::filesystem. Remove Boost Filesystem & System" #20744, I found an unfortunate [pitfall](https://github.com/bitcoin/bitcoin/pull/20744#issuecomment-946046666) -- `boost/process.hpp` depends on `boost::filesystem` when cross-compiling for Windows. It happens in Guix environment as well.

The goal of this issue is to discuss and find a solution to get rid of `boost::filesystem`.
{
    if (fflush(file) != 0) { 
        LogPrintf("%s: fflush failed: %d\n", __func__, errno);
        return false;
    }
#ifdef WIN32
    HANDLE hFile = (HANDLE)_get_osfhandle(_fileno(file));
    if (FlushFileBuffers(hFile) == 0) {
        LogPrintf("%s: FlushFileBuffers failed: %d\n", __func__, GetLastError());
        return false;
    }
#elif defined(MAC_OSX) && defined(F_FULLFSYNC)
    if (fcntl(fileno(file), F_FULLFSYNC, 0) == -1) { 
        LogPrintf("%s: fcntl F_FULLFSYNC failed: %d\n", __func__, errno);
        return false;
    }
#elif HAVE_FDATASYNC
    if (fdatasync(fileno(file)) != 0 && errno != EINVAL) { 
        LogPrintf("%s: fdatasync failed: %d\n", __func__, errno);
        return false;
    }
#else
    if (fsync(fileno(file)) != 0 && errno != EINVAL) {
        LogPrintf("%s: fsync failed: %d\n", __func__, errno);
        return false;
    }
#endif
    return true;
}
void DirectoryCommit(const fs::path &dirname)