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