Clarify container exit logic

Condensed three variables into one variable with three states.  Clarified logic handling these states.
golint:

```
agent.go:387:2: comment on exported const TermExit should be of the form "TermExit ..."
agent.go:390:2: comment on exported const TermSignal should be of the form "TermSignal ..."
agent.go:393:2: comment on exported const TermOOM should be of the form "TermOOM ..."
```