Reap exec sessions on cleanup and removal

We currently rely on exec sessions being removed from the state
by the Exec() API itself, on detecting the session stopping. This
is not a reliable method, though. The Podman frontend for exec
could be killed before the session ended, or another Podman
process could be holding the lock and prevent update (most
notable in `run --rm`, when a container with an active exec
session is stopped).

To resolve this, add a function to reap active exec sessions from
the state, and use it on cleanup (to clear sessions after the
container stops) and remove (to do the same when --rm is passed).
This is a bit more complicated than it ought to be because Kata
and company exist, and we can't guarantee the exec session has a
PID on the host, so we have to plumb this through to the OCI
runtime.

Fixes #4666

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

libpod/container_api.go

@@ -594,7 +594,12 @@ func (c *Container) Cleanup(ctx context.Context) error {
 
 	// If we didn't restart, we perform a normal cleanup
 
-	// Check if we have active exec sessions
+	// Reap exec sessions first.
+	if err := c.reapExecSessions(); err != nil {
+		return err
+	}
+
+	// Check if we have active exec sessions after reaping.
 	if len(c.state.ExecSessions) != 0 {
 		return errors.Wrapf(define.ErrCtrStateInvalid, "container %s has active exec sessions, refusing to clean up", c.ID())
 	}