Allow EventListeners to do PROCESS_COMMUNICATION_STDERR #800
Description
While the stdin and stdout may be blocked for doing event-listener protocol, allow an event-listener to use the stderr stream to send PROCESS_COMMUNICATION_STDERR messages.
This is handy in multiple use-cases in which an eventlistener listening on TICK events can communicate something that it found back to some other event listener.
As of supervisor 3.3.0 this behavior is already allowed by setting stderr_capture_maxbytes to be non-zero.
Requesting this to be a feature.