Fix build and test on Haiku OS

[carmiac]

While working on getting TaskWarrior 3 to build, run, and pass all tests on Haiku I tracked down the last few test failures to libshared.

This PR does a few things:

1. Adds a Haiku CI runner for testing.
2. Add general build support for Haiku
3. Fixes handling interrupted syscalls. This is half a Haiku bug and half a libshared issue. The Haiku issue is that wait() is failing due to a broken default ignore syscall issue, but waitpid() works and is arguably more correct here. The libshared issue is that haiku doesn't do the auto syscall retry that linux does, which isn't technically required under posix but most modern *nix's do. This might fix the same issue on some other, more obscure *nix's but I haven't tested it.
4. Fixes a test for Japanese So unicode symbols. In this case Haiku is technically correct and the test is wrong because the unicode library on linux is whats really wrong, but it's probably too widespread to fix.

[djmitche]

Looks like good fixes to me!

[lauft]

@carmiac The Haiku test fails on master 👉🏻 https://github.com/GothenburgBitFactory/libshared/actions/runs/31892505927/job/95030782508
Can you have a look?

[carmiac]

@lauft Looks like their current infra is having issues due to the upcoming release of beta6. I'll pull the test for now.