Add chvt(1)

This is essentially cherry-picked from sbase with slight
modifications as we can now assume that it will be running on
Linux.

Makefile

@@ -15,6 +15,7 @@ LIB = \
 	util/tty.o
 
 SRC = \
+	chvt.c              \
 	df.c                \
 	dmesg.c             \
 	halt.c              \

chvt.1

@@ -0,0 +1,10 @@
+.TH CHVT 1 ubase\-VERSION
+.SH NAME
+chvt \- change foreground virtual terminal
+.SH SYNOPSIS
+.B chvt
+.I N
+.SH DESCRIPTION
+.B chvt
+brings /dev/ttyN to the foreground. This has the
+same effect as Ctrl-Alt-FN.

chvt.c

@@ -0,0 +1,55 @@
+#include <linux/vt.h>
+#include <linux/kd.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <sys/types.h>
+#include <sys/ioctl.h>
+#include "util.h"
+
+char *vts[] = {
+	"/proc/self/fd/0",
+	"/dev/console",
+	"/dev/tty",
+	"/dev/tty0",
+};
+
+static void
+usage(void)
+{
+	eprintf("usage: chvt N\n");
+}
+
+int
+main(int argc, char **argv)
+{
+	int n, i, fd;
+	char c;
+
+	if(argc!=2 || strspn(argv[1], "1234567890") != strlen(argv[1]))
+		usage();
+
+	n = atoi(argv[1]);
+	for(i = 0; i < LEN(vts); i++) {
+		fd = open(vts[i], O_RDONLY);
+		if(fd < 1)
+			continue;
+		c = 0;
+		if(ioctl(fd, KDGKBTYPE, &c) == 0)
+			goto VTfound;
+		close(fd);
+	}
+
+	eprintf("chvt: couldn't find a console.\n");
+VTfound:
+	if(ioctl(fd, VT_ACTIVATE, n) == -1)
+		eprintf("chvt: VT_ACTIVATE '%d':", n);
+	if(ioctl(fd, VT_WAITACTIVE, n) == -1)
+		eprintf("chvt: VT_WAITACTIVE '%d':", n);
+	close(fd);
+
+	return 0;
+}
+