When stem receives a signal, log useful information

In #30011 (moved), when stem hangs in Tor's CI, we kill it.

But that doesn't give us much information. To fix #29437 (moved), we need stem to log a backtrace when it receives a signal. (Or switch into debug mode, or something.)

You can pick any signal you like: maybe SIGUSR2 for debug logs, and SIGABRT for a stacktrace? (If you want stem to be like tor.)