v.1.2.17: Server exit on SIGINT (ctrl-C)

YAILSRV doesn't have a good exit strategy. This is a heavy handed approach that installs a signal handler for SIGINT (ctrl-C) and exits the application, in whatever state it's in, when it receives the signal.

Need to re-work things so that both the asyncio of the DDG handler is properly told to die as well as the rest of the app.