fix popen thread safety

[pvouzis]

Lock is missing when parsing pidlist which can cause undefined behavior and a seg fault in multithread applications.

Added locks to fix the issue.

[wAuner]

Hey Panos, did you file a bug report with Apple via the Feedback Assistant? This PR will probably never be merged here.

[pvouzis]

Hi @wAuner I had this discussion https://developer.apple.com/forums/thread/726713 and I also wrote this: https://netbeez.net/blog/an-11-year-old-bug-in-the-macos-popen/

If there is a better way to get their attention lmk.

[wAuner]

I'm afraid there's probably nothing more that you could do. It's up to Apple now to fix this. I asked Quinn in your thread about the status.
How did you proceed to work around this issue? Did you replace the popen calls or used any of the other implementations you mentioned?

[pvouzis]

I got the current copy of popen(), I fixed the issue and I used it.