DOS release

RustPython release for DOS

This uses w2c2 to convert RustPython,
built for the wasm32-wasi platform, to C89, which is then compiled with DJGPP and linked to the w2c2 wasi runtime library.
rustpython-nofreeze.exe is built without the freeze-stdlib feature, so it does not have the stdlib.
rustpython-freeze.exe is built with freeze-stdlib, so the standard library is there and usable.
Both need CWSDPMI.EXE, to provide the 32-bit runtime.