os/exec: Process rework

[leongross]

While working on os.StartProcess we realized, that the current process model is (almost) entirely stubbed out.

Retrieving exit status information and other useful information requires a rework of the current process model (right now the only information we keep track of is the pid). To keep this streamlined and simpler than the golang implementation we should discuss the necessary functionality this new process model should provide.

[deadprogram]

@leongross since #4377 was merged can this issue be closed now?