The bare-bones `debian` distribution we use for testing doesn't have
`ps`. This patch switches to reading `/proc` file system directly on
Linux instead of relying on `ps`.
Performance measurements for the 20000 active processes on Debian Docker
container, tested on my M1 Max Mac:
- the `ps -eo pid,ppid,pgid` call + parsing takes 293ms
- the manual synchronous `/proc` traversal + parsing takes 326ms
So this is ~10% perf penalty.
Drive-by: rename `pgid` into `pgrp` so that it stands out from `pid`
(process ID) and `ppid` (parent process ID).
1962b5be3c