readme: create Ctrl-Alt-Del and improve SysRq

kernel_module/user/ctrl_alt_del.c

@@ -0,0 +1,24 @@
+#define _XOPEN_SOURCE 700
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/reboot.h>
+#include <unistd.h>
+
+void signal_handler(int sig) {
+	write(STDOUT_FILENO, "cad\n", 4);
+	signal(sig, signal_handler);
+}
+
+int main(void) {
+	int i = 0;
+	/* Disable the forced reboot, enable sending SIGINT to init. */
+	reboot(RB_DISABLE_CAD);
+    signal(SIGINT, signal_handler);
+	while (1) {
+		sleep(1);
+		printf("%d\n", i);
+		i++;
+	}
+    return EXIT_SUCCESS;
+}

kernel_module/user/poweroff.c

@@ -2,6 +2,7 @@
  * https://stackoverflow.com/questions/28812514/how-to-shutdown-linux-using-c-or-qt-without-call-to-system
  **/
 
+#define _XOPEN_SOURCE 700
 #include <stdio.h>
 #include <sys/reboot.h>
 #include <unistd.h>