Applications don't clean up doors when exiting

[robertdfrench]

Describe the bug
The example applications don't do any signal handling, so they don't clean up their doors if you kill them.

To Reproduce

1. Default portunusd.conf
2. Start any of the example applications (in /opt/local/libexec/portunusd/)
3. kill that application
4. Observe that its door still exists in /var/run/

Expected behavior
An exiting door server should cleanly detach its doors from the filesystem.

[robertdfrench]

reproduction steps are different now (just cargo run --example x) but this problem remains -- there should be some way to:

• correctly detach doors on a clean exit
• Attempt to detach doors on a tumultuous exit
• Check the "liveness" of doors on startup: like, if a file exists at the desired path, and it is a door, attempt to call it? But then that would need a health check of some kind.