Change the way signals are handled

Make it so that the demo can be interrupted while a test is running

demo/demo

@@ -43,10 +43,19 @@ with open(os.devnull, "wb") as f:
 #   m1-m2-m3-m4-m5-m6-m7-m8                       m10
 #     :2::  :4::  :6::  :8::
 
-def disable_signal_on_children(sig):
-    pid = os.getpid()
-    sigint = signal.signal(sig, lambda *x: os.getpid() == pid and sigint(*x))
-disable_signal_on_children(signal.SIGINT)
+# When request_stop is true, subprocess know they need to exit
+# and execute their exit routine
+request_stop = False
+pid = os.getpid()
+sigint = signal.getsignal(signal.SIGINT)
+
+def sigint_handler(*x):
+    global request_stop
+    if os.getpid() == pid:
+        request_stop = True
+        sigint(*x)
+
+signal.signal(signal.SIGINT, sigint_handler)
 
 Node__add_interface = nemu.Node._add_interface
 def _add_interface(node, iface):