* Locking: <play>.lock (unlinkable fcntl style) plus <play>/good

* Locking: <play>.lock (unlinkable fcntl style) plus <play>/good
  sentinel file.  This will avoid conflicts between simultaneous calls
  to with-testbed and setup (and others).  The `with' mode of the userv
  target is enhanced so that the caller can take out the lock and hold
  it will starting and discarding the testbed more than once.