Implement low-level (`OS\`) subprocess API

[fredemmott]

• finalize signature of OS\posix_spawn() and OS\posix_spawnp()
• (in progress, fred) add _OS\dup()
• add _OS\waitpid_async() or similar
• add _OS\shutdown()
• add public APIs:
  • OS\posix_spawn() and OS\posix_spawnp()
  • OS\waitpid_async()
  • OS\shutdown()
  • OS\dup()
  • OS\socketpair() (already in _OS\)

[fredemmott]

add _OS\waitpid_async() or similar

Ideally, I'd want to add _OS\waitid_async(), and expose it as OS\waitid_async() and OS\waitpid_async() - however, waitid() does not exist in macos, despite being part of the specifications for some time.

I think we'd want to just add waitpid_async(), and if later we want process group support, we add _OS\waitpgid_async(), and add an OS\waitid_async() that wraps the two separate functions - unless MacOS adds waitid in the mean time.

... but the design of process group support can be left for later, that's just be brain dumping. For now, let's stick with just waitpid.