mvnd site:run stays running after mvnd process stops

The embedded Jetty instance stays running after the `mvnd` bin process is stopped.

Tested with versions: 1.0.0 and 1.0.1 (currently under vote)

Steps to repro:
Clone https://github.com/apache/maven-site

Run `mvnd site:run`

open `http://localhost:8080/` (everything works fine)
Back in your terminal kill the process: `^C`

Refresh your browser (pages still load)
run `mvnd --stop` to kill the embedded jetty process

Reproduce the issue with `mvnd site:run`, then stop the `mvnd` process with `^C` and verify whether `http://localhost:8080/` still serves pages. Trace the `site:run` process lifecycle and embedded Jetty shutdown handling; done means stopping `mvnd` also makes the embedded Jetty unavailable, without requiring `mvnd --stop`.

Written by the indexing model from the issue text.