make sys/msdos/msdos-cross-compile.sh build msdos NetHack entirely

make sys/msdos/msdos-cross-compile.sh build msdos NetHack entirely

- should work on linux or MacOS to build an msdos zipfile distribution
- no longer requires env variables be set ahead of it because it will set some
  defaults within
- you must have zip and unzip on your system though
- you have to "make fetch-lua" first if you haven't already done that
- script takes care of obtaining the djgpp-cross-compiler etc, then
  uses it to build msdos NetHack
- to clean and rebuild from scratch:
  make -f sys/msdos/Makefile1.cross clean