modules: rename packages/kernel_modules to packages/lkmc

Then inside, split packages/lkmc into kernel_modules and userland,
to keep userland out of the kernel_modules parent path, which makes no
sense.

Copy built modules and userland to the output rootfs overlay.

Document Linux distro tradeoffs.

packages/lkmc/userland/ctrl_alt_del.c

@@ -0,0 +1,26 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#ctrl-alt-del */
+
+#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;
+}