DXB files

Binary drawing interchange files were just plain busted on non-MS-DOS systems. The problems were:

1. Type codes greater than 127 did not work due to some code incorrectly copied from SLIDE.C.
2. An fread was done into an int, resulting in failure on any machine whose ints are not 16 bits.
3. The AutoCAD manual documented .DXB files as being in Intel byte order, but the code did not perform the required reversals.

I modified all I/O within DXBIN.C to use the Binary I/O package, and corrected these problems. All systems now read DXB files which are compatible with existing MS-DOS files. Since the existing code in non-MS-DOS systems could never have worked, compatibility with existing non-MS-DOS DXB files is not a consideration since none exist.