Win32: Use FILE* streams, not file descriptors

When reading or writing a DFBuffer from or to the filesystem, use fopen,
fread, fwrite, and fclose instead of open/creat, read, write, close.
There were some permission-related problems caused when creating new
files, and the Win32 documentation recommends the former over the
latter.

In other places where we already use FILE* streams, make sure the files
are opened in binary mode; we don't want any line-ending conversion
taking place (at least, not here).