bug#18230: Removing support for DJGPP < 2.02

[Eli Zaretskii]

> Date: Sat, 9 Aug 2014 14:33:02 +0100
> From: Reuben Thomas <rrt@sc3d.org>
> 
> Given that DJGPP 2.02 was released in 1998, may I remove the two files
> which are added to support earlier releases, that is:
> 
> msdos/is_exec.c
> msdos/sigaction.c

OK.  But if we are dropping v2.01, please also remove the code in
msdos.c and dosfns.c, and also in msdos/sed2v2.inp, that caters to
v2.01 and older versions.

We should also mention in msdos/INSTALL than only version 2.02 and
later are supported.

Finally, config.bat should have its test of the DJGPP version updated
to fail v2.01 and older versions.

TIA