Add halt(8)

This is a very barebones halt cmd.  Please make sure that your
system is in a proper state to halt before using this.  This is
likely only useful for very barebones systems like an emergency
shell or similar.

Normally this command would be part of an actual init system.

Makefile

@@ -18,6 +18,7 @@ SRC = \
 
 ifeq ($(OS),linux)
 SRC += \
+	halt.c              \
 	insmod.c            \
 	lsmod.c             \
 	mkswap.c            \

halt.c

@@ -0,0 +1,40 @@
+/* See LICENSE file for copyright and license details. */
+#include <linux/reboot.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+#include <stdio.h>
+#include "util.h"
+
+static void
+usage(void)
+{
+	eprintf("usage: %s [-p]\n", argv0);
+}
+
+int
+main(int argc, char *argv[])
+{
+	int pflag = 0;
+	int cmd = LINUX_REBOOT_CMD_HALT;
+
+	ARGBEGIN {
+	case 'p':
+		pflag = 1;
+		break;
+	default:
+		usage();
+	} ARGEND;
+
+	if (argc > 0)
+		usage();
+
+	sync();
+
+	if (pflag)
+		cmd = LINUX_REBOOT_CMD_POWER_OFF;
+
+	if (syscall(__NR_reboot, 0xfee1dead, 672274793,
+		    cmd, NULL) < 0)
+		eprintf("reboot:");
+	return 0;
+}