Enhanced signal handler

• Moved the signal handler action from main program to the signal handler itself so that the program can directly react.
• New option "-b / --start-blocked" to started with targets blocked regardless of initial condition check.
• Repalced subprocess.call()with new subprocess.run()
  • Removed local function call(), using subprocess.run() directly

Python 3 version

First release of Python 3 upgrade and code clean-up.