Hi,
I found Launcher.ProcStarter.readStderr() has no effect when developing a Jenkins plugin.
After calling `readStderr`, the stderr of the remote process is still merged to `stdout` while `Proc.getStderr()` will always get a stream with empty content.

I currently don't have time to look at the code carefully or fix it, but I guess it is caused by the following reason:

What readStderr() does is just setting `reverseStderr` to true, resulting in the err parameter passed to LocalProc is set to LocalProc.SELFPUMP_OUTPUT (
https://github.com/jenkinsci/jenkins/blob/stable-2.138/core/src/main/java/hudson/Launcher.java#L934), otherwise err parameter will be set to null. But the construct of LocalProc will set the redirectError parameter of ProcessBuilder to true in both cases (https://github.com/jenkinsci/jenkins/blob/stable-2.138/core/src/main/java/hudson/Proc.java#L219).

If my understanding is correct, the `redirectError` parameter should be set to true only when `err == null`.