ANSI C signal handling

Common usage imply 5 steps:

1. Implement the SignalHandler interface
2. redef receive_signal_unsafe to handle sigsegv
3. redef receive_signal to handle other signals safely 4, Notify what signals to handle with handle_signal
4. If using the safe handler method, routinely call check_signals

Usage example:

class MyReceiver
    super SignalHandler

    redef fun receive_signal(signal)
    do
        print "received safely {signal}"
        if signal == sigalarm then print "Alarm!"
    end
    redef fun receive_signal_unsafe( signal ) do print "received unsafely {signal}"
end

var r = new MyReceiver

# Handle `sigsegv` signal unsafely (the only way for this one)
r.handle_signal(sigsegv, false)

# Handle `sigint` and `sigalarm` safely
r.handle_signal(sigint, true)
r.handle_signal(sigalarm, true)

# Ask system to receive a `sigalarm` signal in 1 second
set_alarm(1)

loop
    # Check signals and callback `receive_signal`
    var hit = check_signals

    if hit then break
end