Fix atexit not working on SIGTERM

[blurHY]

I am working on HorizonManager that manages the process of HorizonSpider and zeronet.
It would terminate zeronet by sending SIGTERM.
However, atexit won't work on SIGTERM.We have to manually call sys.exit to trigger atexit.

[HelloZeroNet]

Looking great, but please remove the unnecessary import of atexit and use import signal signal.signal(signal.SIGTERM, lambda signum, stack_frame: sys.exit(0)) to reduce global namespace variables

[radfish]

Zeronet 0.6.5 just hangs on receiving SIGTERM from systemd.
Maybe sys.exit(0) from signal handler is not good? Perhaps something needs to be stopped from the main loop? I'll look into it if I find the time.

[blurHY]

sys.exit would only raise a SystemExit exception. Non-daemon threads won't exit