Adding checks for fd omission

Adding function check_fds to new file fd.c. The function check_fds
should be called in every setuid/setgid program.

Co-developed-by: Alejandro Colomar <alx@kernel.org>

src/chsh.c

@@ -472,6 +472,7 @@ int main (int argc, char **argv)
 	const struct passwd *pw;	/* Password entry from /etc/passwd   */
 
 	sanitize_env ();
+	check_fds ();
 
 	log_set_progname(Prog);
 	log_set_logfd(stderr);