API for spawning sub-processes

[talex5]

Eio needs to provide a way to create and manage sub-processes (like Lwt_process).

• Must work in programs using multiple domains. Note that Unix.fork cannot be used there.
• API should try to prevent bugs with argument splitting (e.g. avoid error-prone Unix.system style API by default). But also needs to work on Windows, where I think you can't provide an argv.
• Child processes should be attached to a switch by default to avoid leaking them, but need a way to spawn detached sub-processes too.
• Must allow passing pipes, etc, to child processes.
• Must prevent accidentally leaking FDs to child processes. All FDs in Eio are already opened with close-on-exec set for this reason.
• Should provide a simple way to check or report the process's exit status.

Possible sources of inspiration include:

• Lwt_process
• Bos.Cmd
• Feather
• Python's subprocess

Current status:

• eio_luv: add low-level process support #359
• eio_posix: initial support for subprocesses #461
• Add inherit_fds fork action #464
• eio_linux: add subprocess support #472
• Cross-platform subprocess support #499

[patricoferris]

Just a heads up that I've started looking into this as I'm going to need it for some applications I'm building. Hopefully will have a draft PR in the next week or two to iron out the details :))

[benmandrew]

Polite bump; this would be extremely useful for converting a lot of the ocurrent ecosystem to Eio. The work in ocurrent/obuilder#113 looks great so far!

[talex5]

The current PR (submitted 2 days ago) is here: #473

[talex5]

Implemented in #499.