Return exit status from Wait of stopped process

This changes Wait() from returning an error whenever you call wait on a
stopped process/task to returning the exit status from the process.

This also adds the exit status to the Status() call on a process/task so
that a user can Wait(), check status, then cancel the wait to avoid
races in event handling.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

checkpoint_test.go

@@ -264,7 +264,7 @@ func TestCheckpointLeaveRunning(t *testing.T) {
 		t.Error(err)
 		return
 	}
-	if status != Running {
+	if status.Status != Running {
 		t.Errorf("expected status %q but received %q", Running, status)
 		return
 	}