MEDIUM: listener: let do_unbind_listener() decide whether to close or not

MEDIUM: listener: let do_unbind_listener() decide whether to close or not

The listener contains all the information needed to decide to close on
unbind or not. The rule is the following (when we're not stopping):

  - worker process unbinding from a worker's FD with socket transfer enabled => keep
  - master process unbinding from a master's inherited FD => keep
  - master process unbinding from a master's FD => close
  - master process unbinding from a worker's FD => close
  - worker process unbinding from a master's FD => close
  - worker process unbinding from a worker's FD => close

Let's translate that into the function and stop using the do_close
argument that is a bit obscure for callers. It was not yet removed
to ease code testing.