Offer: PR for CHOICE support, Unix build fixes, runtime bug fixes, and regression tests

[ostrich]

I started out just wanting to add CHOICE support to Picobat so I could run a batch Tetris script under it. That turned into a larger cleanup pass because each step exposed another real issue.

Progression was roughly:

1. I added a CHOICE builtin.
2. While trying to build/test it on Linux, I hit build/bootstrap problems, so I fixed the Unix config detection and build wiring first.
3. Once the tree built, I cleaned up a number of compiler warnings and portability issues. Some of those also turned out to be real bugs, not just warning noise.
4. I then expanded the test coverage substantially.
5. Running those tests exposed several runtime bugs in pipes, redirection, COPY, CALL, IF ERRORLEVEL, etc., so I fixed those too.

I used Codex heavily during this work, both to help inspect the codebase and to iteratively implement/test fixes.

Current change summary:

• add a CHOICE builtin with /C, /N, /CS, /D, /T, /M, timeout/default handling, and DOS-style /T:X,1
• fix Unix build/config detection and build ordering issues
• fix a number of portability/type/header issues across the codebase
• fix several real runtime bugs:
  • pipe/input/output handling
  • redirection truncation vs append
  • COPY /Y
  • CALL behavior for call set ... and call %0 label ...
  • IF ERRORLEVEL
  • helper dump bugs used by tests
• add a much larger regression test suite

If this sounds useful, I’m happy to submit it as a PR against the dev branch.

[TSnake41]

Feel free to make a PR. I will try to consider reviewing it.