Feature request: --json-status-fd could provide pid of command?

`--json-status-fd` gives a `child-pid` value, which is handy. In a lot of situations that is the pid of the COMMAND you're running in the sandbox.

But when run with `--unshare-pid` (and maybe others) `bwrap` forks itself and `child-pid` references the fork. I would like a way of getting the pid of the COMMAND. Could this be added to the JSON output?

Thanks.