fix: adjust close_handles pointer offsets to match distlib cleanup_fds

[samypr100]

Summary

Resolves issues mentioned in comments

• Stubs on Windows exit abnormally, includes succinct repro with busybox, related to some wider issue #6699 (comment)
• CMake stub crashes when executed by VS Code #6866 (comment)

Further investigation on the comments revealed that the pointer arithmethic being performed in let handle_start = unsafe { crt_magic.offset(1 + handle_count) }; from posy trampoline had some slight errors. Since crt_magic was a *const u32, doing an offset by 1 + handle_count would offset by too much, with some possible out of bounds reads or attempts to call CloseHandle on garbage.

We needed to offset differently since we want to offset by handle_count bytes after the initial offset as seen in launcher.c. Similarly, we needed to skip the first 3 handles, otherwise we'd still be attempting to close standard I/O handles of the parent (in this case the shell from busybox.exe sh -l).

I also added a few extra checks available from launcher.c which checks if the handle value is -2 just to match the distlib implementation more closely and minimize differences.

Test Plan

Manually compiled distlib's launcher with additional logging and replaced Lib/site-packages/pip/_vendor/distlib/t64.exe with the compiled one to log pointers. As a result, I was able to verify the retrieved handle memory addresses in this function actually match in both uv and distlib's implementation from within busybox.exe nested shell where this behavior can be observed and manually tested.

I was also able to confirm this fixes the issues mentioned in the comments, at least with busybox's shell, but I assume this would fix the case with cmake.

Open areas

launcher.c also checks the size of cbReserved2 before retrieving handle_start which this function currently doesn't do. If we wanted to, we could add the additional check here as well, but I wasn't fully sure why it wasn't added in the first place. Thoughts?

// Verify the buffer is large enough
if si.cbReserved2 < (size_of::<u32>() as isize + handle_count + size_of::<HANDLE>() as isize * handle_count) as u16 {
    return;
}

[samypr100]

CC @konstin