Provide `posix_spawn`

[hasufell]

https://man7.org/linux/man-pages/man3/posix_spawn.3.html

It's supposed to be the successor of fork. Unless I missed something, it doesn't appear to be bound here.

[hasufell]

It seems the process package has some C code that could be salvaged:

https://github.com/haskell/process/blob/e448450049ac51a6fcd5d1f0be86448d0931d71a/cbits/posix/posix_spawn.c#L202

[hs-viktor]

Reasonable request, Cc'ing @bgamari . Ben your thoughts.

[hasufell]

@merijn