Currently, the new API will only be available on macOS with
the intention of adding more POSIX systems to it incrementally
(such as Linux, etc.).
Changes:
* add `posix_spawn` wrappers in a separate container in
`os/posix_spawn.zig`
* rewrite `ChildProcess.spawnPosix` using `posix_spawn` targeting macOS
as `ChildProcess.spawnMacos`
* introduce a `posix_spawn` specific `std.c.waitpid` wrapper which
does return an error in case the child process failed to exec - this
is required for any process that was spawned using `posix_spawn`
mechanism as, by definition, the errors returned by `posix_spawn`
routine cover only the `fork`-equivalent; `pre-exec()` and `exec()`
steps are covered by a catch-all error `ECHILD` returned by `waitpid`
on unsuccessful execution, e.g., no such file error, etc.
