This makes `runsc wait` behave more like waitpid()/wait4() in that:
- Once a process has run to completion, you can wait on it and get its exit
code.
- Processes not waited on will consume memory (like a zombie process)
PiperOrigin-RevId: 213358916
Change-Id: I5b5eca41ce71eea68e447380df8c38361a4d1558