Add a mostly disabled feature to debug restarting in-process
For 23847, we want Tor to be able to shut down and then restart in the same process. Here's a patch to make the Tor binary do that. To test it, you need to build with --enable-restart-debugging, and then you need to set the environment variable TOR_DEBUG_RESTART. With this option, Tor will then run for 5 seconds, then restart itself in-process without exiting. This only happens once. You can change the 5-second interval using TOR_DEBUG_RESTART_AFTER_SECONDS. Implements ticket 24583.