More elegant way of solving #5839 (d_private in sceIoDread)

We flag filesystems as being FAT32 instead of checking for "ms0:".
VFSFileSystem too
Make strcpy_limit slightly more sane.
Merge pull request #5871 from hrydgard/sceiodread_ms0check

More elegant way of solving #5839 (d_private in sceIoDread)