Support graceful stop

Currently, `mvnd --stop` will stop all background tasks.

We should, like `ssh -O` option's behavior:
* use `mvnd --stop` as "request the master to stop accepting further request"
* use `mvnd --exit` as "request the master and tasks to exit"

Start at the command handling for `mvnd --stop` and trace how the master and background tasks are currently stopped. Compare the desired `--stop` and `--exit` behaviors with the existing shutdown flow; done means `--stop` rejects new requests while allowing tasks to finish, and `--exit` stops both the master and tasks.

Written by the indexing model from the issue text.