Import vDos Version 2014.03.14

The master source code release for vDos Version 2014.03.14 with some of
the binaries removed (the original was bloated by a few extraneous
binaries which were removed from later release sets).
Import vDos Version 2014.04.06

The master source code release for vDos Version 2014.04.06 with some of
the binaries removed (the original was bloated by a few extraneous
binaries which were removed from later release sets).
Added support for DOS Mailslot functions

This adds support for INT 21 functions 5F4D (DosMakeMailslot), 5F4E
(DosDeleteMailslot), 5F4F (DosMailslotInfo), 5F50 (DosReadMailslot), and
5F52 (DosWriteMailslot). It does NOT support 5F51 (DosPeekMailslot).
Change boot drive from Z: to A:

The internal boot drive can now be changed by modifying two defines in
dos_system.h.
Merge remote-tracking branch 'origin/bootdrive'