MS-DOS 1.20

According to MS-DOS 1.25 and 2.00 source code, MS-DOS 1.20 is an unreleased internal version of MS-DOS.

Change log
1.20 12/31/81 Add time to FCB; separate FAT from DPT; Kill SMALLDIR; Add FLUSH and MAPDEV calls; allow disk mapping in DSKCHG; Lots of smaller improvements