feat(chvt): use custom implementation instead of system()

Calling system() is vulnerable and shouldn't be used.

src/chvt.c

@@ -0,0 +1,22 @@
+#include "chvt.h"
+
+static char *vterms[] = {"/dev/tty", "/dev/tty0", "/dev/vc/0", "/dev/systty",
+                         "/dev/console"};
+
+int chvt(int n) {
+  char c = 0;
+  for (int i = 0; i < sizeof(vterms) / sizeof(vterms[0]); i++) {
+    int fd = open(vterms[i], O_RDWR);
+    if (fd >= 0 && isatty(fd) && ioctl(fd, KDGKBTYPE, &c) == 0 && c < 3) {
+      if (ioctl(fd, VT_ACTIVATE, n) < 0 || ioctl(fd, VT_WAITACTIVE, n) < 0) {
+        printf("Couldn't active vt %d\n", n);
+        return -1;
+      }
+      return 0;
+    }
+    close(fd);
+  }
+
+  printf("Couldn't get a file descriptor referring to the console.\n");
+  return -1;
+}