fix compilation with new new nightly VaList

[LGFae]

In 2025-12-05, a Rust pull request at rust-lang/rust#141980 reworked the VaList implementation to be abi-compatible with C. This broke:

• the printf-compat implementation, which is fixed in version 0.3
• some fcntl functions, because they called as_va_list, and that function no longer exists

Here, we fix both of those problems and update the compiler version in rust-toolchain.toml to be the oldest compiler that will already have this new implementation.

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Fixes #164

[LGFae]

My bad, the rust version was off by 1 day.

[LGFae]

Sigh. My apologies. I was just running cargo check, blindly believing that was enough because the semantics shouldn't have changed. But apparently it doesn't type check the tests.

Unfortunately, I have to do something else for now. I will fix this later today.

[LGFae]

Third time's the charm???

In any case, I've now fixed what I believe to be every function that was using VaList. At the very least, neither cargo check nor cargo test accuse any more problems.

Beside that, as I explain in the commit message, updating the compiler version created 2 unforseen problems:

1. When unwinding, the new Rust version now prints the process's PID.
   This means for the c-gull-unwinding we can no longer use the assert_cmd API, as it does not expose the child PID to us.
2. The c-gull/src/nss.rs had 3 extra warnings of operations whose results were unused. This was also breaking some other tests that seem to compare the build's output. Those 3 useless statements were deleted and now the tests pass

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It seems I underestimated how much work this was. Hopefully it won't fail miserably at literally the very first check this time 😅

[sunfishcode]

This looks good. Thanks!