Ensure Python app can handle SIGTERM/SIGKILL

[halajohn]

Go app had special handling for these signals.

[sunxilin]

The current status is that TEN Python app can handle SIGTERM/SIGKILL as the TEN signal handler is registered after the library 'libten_runtime' is loaded. The performance is that 'ctrl+C' will trigger the TEN app gracefully close.

[halajohn]

The signal handler registered in ten_runtime's on_load will not be overwritten in the Python environment. Therefore, in a Python TEN app, pressing Ctrl-C once will stop the app, and pressing Ctrl-C twice will forcibly exit the TEN app. Both behaviors are effective.