Fix signal handler

[elfring]

The functions "exit" and "fclose" do not belong to the list of async-signal-safe functions. I guess that a different program design will be needed for your function "signal_callback_handler".

[andy-shev]

I suppose atexit() should work well for the expected purpose.

[elfring]

Would you like to consider other software design options?

• sigwaitinfo()
• Self-pipe trick