use pledge().

Index: main.c
--- main.c.orig
+++ main.c
@@ -173,6 +173,7 @@
 #include <string.h>
 #include <ctype.h>
 #include <assert.h>
+#include <unistd.h>
 
 #if defined(unix) && !defined(GO32)
 #include <signal.h>
@@ -231,6 +232,11 @@ void usage(FILE *fp)
  * Main program
  */
 int main(int argc, char **argv) {
+    if (pledge("stdio rpath wpath cpath tty", NULL) == -1) {
+        fprintf(stderr, "pledge\n");
+        return 1;
+    }
+
     fileoffset_t newoffset = -1, newwidth = -1;
 
     /*