Add vtallow(1) to enable or disable VT switch

Makefile

@@ -82,6 +82,7 @@ BIN = \
 	umount            \
 	unshare           \
 	uptime            \
+	vtallow           \
 	watch             \
 	who
 

vtallow.c

@@ -0,0 +1,52 @@
+/* See LICENSE file for copyright and license details. */
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/ioctl.h>
+
+#include <fcntl.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "util.h"
+
+#define CONSOLE "/dev/console"
+
+#define VT_LOCKSWITCH   0x560B  /* disallow vt switching */
+#define VT_UNLOCKSWITCH 0x560C  /* allow vt switching */
+
+static void
+usage(void)
+{
+	eprintf("usage: vtallow n|y\n");
+}
+
+int
+main(int argc, char *argv[])
+{
+	int fd;
+	int allow;
+
+	ARGBEGIN {
+	default:
+		usage();
+	} ARGEND;
+
+	if (argc != 1)
+		usage();
+
+	if (!strcmp(argv[0], "y"))
+		allow = 1;
+	else if (!strcmp(argv[0], "n"))
+		allow = 0;
+	else
+		usage();
+
+	if ((fd = open(CONSOLE, O_WRONLY)) < 0)
+		eprintf("open %s:", CONSOLE);
+	if (ioctl(fd, allow ? VT_UNLOCKSWITCH : VT_LOCKSWITCH) < 0)
+		eprintf("cannot %s VT switch:",
+			allow ? "enable" : "disable");
+	close(fd);
+	return 0;
+}