[illumos] attempt to use posix_spawn to spawn processes illumos has `posix_spawn`, and the very newest versions also have `_addchdir`, so use that. POSIX standardized this function so I also added a weak symbol lookup for the non `_np` version. (illumos has both.) This probably also works on Solaris, but I don't have access to an installation to validate this so I decided to focus on illumos instead. 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]: https://github.com/clap-rs/clap/commit/fde45f9aea766fb8de46e3d46e6575f393c3b6b9