This is a meta enhancement request to add the needed infrastructure for multi-process debugging in general.

Here are some points that we need to address:

* Support asynchronous GDB/MI notification of threads lifetime related events.
* Track a stack of forked processes starting from the current inferior. I.e, when the current inferior does a fork, we should be able to record the environment of the child process, but we should also be able to keep the environment of the parent process around, in case the user e.g. would want to restart either the child or the parent with its initial environment.
* Support non-stop debugging for threads.