Allow self-destruct via "kill -s"

HTTP(s) payloads don't exit cleanly at the moment. This is an issue that's
being addressed through other work. However, there's a need to be able to
terminate the current HTTP(s) session forcably.

This commit add a -s option to kill, which (when specified) will kill
the current session.
Land #2729 - Allow manual self-destruct via "kill -s"