Rain b340545114 [illumos] attempt to use posix_spawn to spawn processes
illumos has `posix_spawn`, and the very newest versions also have `_addchdir`,
so use that.

This is a nice ~4x performance improvement for process creation. My go-to as
usual is nextest against the clap repo, which acts as a stress test for process
creation -- with [this commit]:

```console
$ cargo nextest run -E 'not test(ui_tests) and not test(example_tests)'
before: Summary [   1.747s] 879 tests run: 879 passed, 2 skipped
after:  Summary [   0.445s] 879 tests run: 879 passed, 2 skipped
```

[this commit]: fde45f9aea
2025-02-23 06:49:59 +00:00
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