Option to run propolis-runner in its own process group (Setpgid)

[JAORMX]

Summary

Currently, propolis-runner is spawned as a child process in the same process group as the calling process. This means:

• A SIGINT from a terminal (Ctrl+C) sends the signal to the entire process group, so both the caller and propolis-runner receive it — which may or may not be desired.
• A targeted SIGKILL on just the caller process leaves propolis-runner as an orphan in the parent's process group, with no easy way to clean it up by group.

Requested change

Add an option (e.g. WithRunnerProcessGroup() or as part of a RunnerOptions) that sets Setpgid: true in SysProcAttr when spawning propolis-runner. This places the runner in its own process group, giving the caller a stable group ID to target when cleaning up:

// Caller can then do:
syscall.Kill(-pgid, syscall.SIGTERM)

Alternatively, expose the process group ID via a method or the data dir so the caller can use it for targeted cleanup.

Use case

apiary needs to clean up orphaned propolis-runner processes after a crash. With the runner in a dedicated process group, startup cleanup can send SIGTERM to the entire group (runner + any of its own children) safely, without affecting unrelated processes in the parent's group.

Notes

• This option should be opt-in, not default, to avoid breaking existing callers that rely on the current process group behavior.
• On macOS, Setpgid works the same way via syscall.SysProcAttr.