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 forcefully in certain cases.

This PR add a `-s` option to kill, which (when specified) will kill the current session. This makes session on the MSF side hang, but that can be terminated manually.