Launch process without stdin by default

[rgrinberg]

Introduce a null input/output for processes. This can be used for
redirecting to /dev/null or running a process without stdin.

Change the default for Process.run to not read anything from stdin.
Previously, processes would inherit stdin from dune. That seems like a
bad default.

[rgrinberg]

Added @aalekseyev as a reviewer since he expressed interest in the PR.

[aalekseyev]

I think action_exec.ml also needs a fix: it uses Process.Io.stdin where it should use the null device. In fact I would imagine that's the only place that's important to fix because that's used to run user-specified actions.

I'm now confused: is your original fix intended to affect something that's not an action, or was it just not tested?

[aalekseyev]

By the way, that's the only remaining use of Process.Io.stdin, so we should be able to delete that.

[rgrinberg]

I think action_exec.ml also needs a fix: it uses Process.Io.stdin where it should use the null device. In fact I would imagine that's the only place that's important to fix because that's used to run user-specified actions.

Good call. Pushed a commit to fix this.

I'm now confused: is your original fix intended to affect something that's not an action, or was it just not tested?

My fix was meant to address #3672. So this change was sufficient to fix the cram action in this PR #3601

By the way, that's the only remaining use of Process.Io.stdin, so we should be able to delete that.

I think this is still going to be useful for #3660 so I'll leave it for now

[aalekseyev]

Looks good.
A small nit: it would probably make sense to have the special case | File fn1, File fn2 when Path.equal fn1 fn2 in command_line_enclosers function in process.ml to also apply in the Null, Null case.

Maybe one way to do it is to add:

let io_to_redirection_path (io : _ Io.t) = match io.kind with
  | Terminal -> None
  | Null -> Some (Lazy.force Io.null_path)
  | File fn -> Some (Path.to_string fn)

But feel free to do whatever you find expedient here.

I also wonder if we should add a changelog entry for this, since it's a behavior change that can be noticed by users in rare cases. (clearly when the command is blocked on stdin, but more subtly that can happen if the command uses stdin to determine the terminal dimensions or to decide whether or not colors should be used, etc.)