Save and restore errno

[hanna-kruppe]

Closes #191

Based on #193 for now. I can rebase on master once CI if green if you want to merge this first.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 93.75000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.54%. Comparing base (172283a) to head (34a357a).
⚠️ Report is 43 commits behind head on master.

Files with missing lines	Patch %	Lines
signal-hook-registry/src/lib.rs	93.10%	2 Missing ⚠️
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Additional details and impacted files

@@             Coverage Diff             @@
##           master     #194       +/-   ##
===========================================
+ Coverage   60.20%   73.54%   +13.34%     
===========================================
  Files          16       17        +1     
  Lines         784     1119      +335     
  Branches      117        0      -117     
===========================================
+ Hits          472      823      +351     
- Misses        241      296       +55     
+ Partials       71        0       -71     

Files with missing lines	Coverage Δ
serial_test/src/lib.rs	100.00% <100.00%> (ø)
signal-hook-tokio/src/lib.rs	93.93% <ø> (-6.07%)	⬇️
src/iterator/mod.rs	77.04% <ø> (ø)
src/low_level/mod.rs	50.00% <ø> (+10.00%)	⬆️
signal-hook-registry/src/lib.rs	86.38% <93.10%> (+8.98%)	⬆️

... and 13 files with indirect coverage changes

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[hanna-kruppe]

@vorner While checking if the test I added does anything, I seem to have discovered that CI was never running on Windows to begin with because ci-check.sh was run under powershell instead of bash 😬 signal-hook's C code for extracting signal details supposed to work on Windows? Because CI is reporting a ton of MSVC errors: https://github.com/vorner/signal-hook/actions/runs/19617203886/job/56171562035?pr=193#step:5:127

[vorner]

Looks nice overall :-). Let's wait for the other branch first, then.

signal-hook's C code for extracting signal details supposed to work on Windows?

I think that one was never supposed to run on Windows - I think that platform doesn't provide the extended information about signals and has only very minimal support for them. I'd say the best solution there is just to not enable that feature for Windows (I suspect that was actually the biggest reason why that's under a feature flag, but it's a long time since I wrote it).