MSDOS support

[Dan Nicolaescu <dann@ics.uci.edu>]

Emacs running on MSDOS was probably a technical wonder (Eli and all
other people that made it possible deserve kudos for this excellent
work), and probably it was very important at the time.  But at this time
supporting this platform does not seem useful anymore.

Wouldn't it be a good idea to remove the MSDOS support?

The main reason is that there is no evidence that there are any emacs
users still using this proprietary platform.

The MSDOS support has been broken in CVS for almost one year.  There
were no complaints from any users.

The removal of the MSDOS support was announced last year and widely
publicized in the first round of old platform removals.  We received no
comments from any user about MSDOS, so RMS formally approved the
removal.  It was not removed because Eli said he _might_ want to make it
run again.

The impact of MSDOS on the code is very high, there are hundreds or
#ifdef MSDOS/DJGPP/GO32 just in the src directory.  Not having those
#ifdefs would simplify the code and make it easier to read and maintain.

MSDOS is the only supported platform that does not define
"subprocesses", that's another chunk of code that is special-cased for
MSDOS.

There is an active burden from keeping this non-working port in the
tree: file names need to be chopped to fit into the 8.3 format (last
time was just one week ago).

There's no guarantee that MSDOS code will ever work again, but given the
absence of users, it does not seem something worthwhile doing, so better
get rid of if sooner rather than later.