Prune stale exec sessions on inspect

The usual flow for exec is going to be:
- Create exec session
- Start and attach to exec session
- Exec session exits, attach session terminates
- Client does an exec inspect to pick up exit code

The safest point to remove the exec session, without doing any
database changes to track stale sessions, is to remove during the
last part of this - the single inspect after the exec session
exits.

This is definitely different from Docker (which would retain the
exec session for up to 10 minutes after it exits, where we will
immediately discard) but should be close enough to be not
noticeable in regular usage.

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

libpod/container_exec.go

@@ -104,7 +104,7 @@ func (e *ExecSession) Inspect() (*define.InspectExecSession, error) {
 	}
 
 	output := new(define.InspectExecSession)
-	output.CanRemove = e.State != define.ExecStateRunning
+	output.CanRemove = e.State == define.ExecStateStopped
 	output.ContainerID = e.ContainerId
 	if e.Config.DetachKeys != nil {
 		output.DetachKeys = *e.Config.DetachKeys