TruncateFile aborts on MSYS/Cygwin

[guberathome]

I could not find an obvious way to fix #189.

This patch aborts RESIZE-FILE (for shrinking) on MSYS2/Cygwin instead of silently appending to the existing file.
Which is just a tiny bit better.

If there should be nothing (else) we could do about it, I would advocate documenting the behaviour
(e.g. in README.md),

Hints are welcome.

Output for make test on {Linux, FreeBsd,NetBSD} is now:

cd ../../fth && ../platforms/unix/pforth_standalone -q t_file.fth
Include t_tools.fth
...
  94 passed,    0 failed.
PForth Tests PASSED

... and on MSYS2/Cygwin:

Include t_tools.fth
    include added 1712 bytes,38292 left.
(got=-74, expected=0) INCORRECT RESULT: T{ 37. FID2 @ RESIZE-FILE -> 0 }T
(got=50, expected=37) INCORRECT RESULT: T{ FID2 @ FILE-SIZE -> 37. 0 }T
(got=50, expected=37) INCORRECT RESULT: T{ CBUF BUF 100 FID2 @ READ-FILE -> 37 0 }T
(got=-1, expected=0) INCORRECT RESULT: T{ PAD 38 BUF 38 S= -> FALSE }T
Include t_required_helper1.fth
    include added 0 bytes,37172 left.
Include t_required_helper1.fth
    include added 0 bytes,37172 left.
Include t_required_helper2.fth
    include added 0 bytes,37172 left.
Include t_required_helper2.fth
    include added 0 bytes,37172 left.
  90 passed,    4 failed.
make: *** [Makefile:165: test] Error 40

[philburk]

Thanks for this PR.
Failing silently was a problem.
This is a step in the right direction.

[guberathome]

done, ready to be merged.

[guberathome]

alternative solution using Unix-API implemented (separate PR).