listen: handle KeyboardInterrupt

Current response to Ctrl+C with "pelican --listen" is to bail with an
exception.

Instead, exit proper and show a message.

Fixes #2676

pelican/__init__.py

@@ -474,6 +474,9 @@ def listen(server, port, output, excqueue=None):
             excqueue.put(traceback.format_exception_only(type(e), e)[-1])
         return
 
+    except KeyboardInterrupt:
+        logger.info("Shutting down server.")
+        httpd.socket.close()
 
 def main(argv=None):
     args = parse_arguments(argv)